self-assembly and surface-patterning of polymer ......ii self-assembly and surface-patterning of...
TRANSCRIPT
Self-Assembly and Surface-Patterning of Polymer-
Functionalized Nanoparticles
by
Rachel Choueiri
A thesis submitted in conformity with the requirements
for the degree of Doctor of Philosophy
Graduate Department of Chemistry
University of Toronto
© Copyright by Rachel Choueiri, 2016
ii
Self-Assembly and Surface-Patterning of Polymer-Functionalized Nanoparticles
Rachel Choueiri
Doctor of Philosophy
Graduate Department of Chemistry
University of Toronto
2016
Abstract
This thesis explores the preparation of polymer-functionalized nanoparticles, the surface-
segregation of their polymer ligands to generate patchy nanocolloids, and the use of patchy and
non-patchy polymer-functionalized nanoparticles as building blocks for self-assembly.
In Chapter 3, the self-assembly of nanospheres uniformly functionalized with polymer
was explored. Both chain-like and globular structures were generated from the same spherical
nanoparticle building blocks by tuning the interplay of nanoscale forces. The molecular weight
of polymer ligands and the polarity and ionic strength of the self-assembly medium were
changed to adjust the interparticle hydrophobic attraction and electrostatic repulsion in order to
obtain the desired self-assembled structure. The experimental results were in agreement with
theoretical predictions for the most thermodynamically favourable self-assemblies. Hierarchical
structures made from assembled globules of nanospheres were also generated and the
reversibility of the self-assemblies was demonstrated.
iii
In Chapter 4, the segregation of polymer ligands into pinned micelles on the surface of
nanospheres was explored. Single and multi-patch nanospheres were generated by studying the
interplay of nanosphere curvature and polymer molecular weight. The statistics of patchy
nanosphere species was quantified and compared to theoretical predictions. Permanent
crosslinking of polymer patches was achieved in addition to tomography of the generated
colloids to further characterize the morphology.
In Chapter 5, the surface segregation of polymer ligands on nanoparticles with different
shapes and composition such as spherocylindrical nanorods, nanorods with a dumbbell shape,
and nanocubes was demonstrated. The surface segregation of different polymers (including pH-
responsive and conductive polymers) on nanospheres was also achieved demonstrating the
versatility of the polymer segregation approach for nanopatterning. In addition, the self-assembly
of patchy nanospheres and nanocubes was explored yielding new self-assembled structures such
as an open checkerboard formed from patchy nanocubes. The use of one-patch nanospheres as
colloidal surfactants stabilizing a solvent-polymer interface was also achieved.
iv
Acknowledgments
First I would like to thank my supervisor and mentor Professor Eugenia Kumacheva for
all her support, insight, and guidance. Her indefatigable optimism, astute suggestions, and kind
advice throughout this journey kept me going. Thank you Eugenia for being such an excellent
supervisor!
Second, I would like to thank my collaborators for their hard work and insight: Prof.
Milad Abolhasani, Prof. Aftab Ahmed, Dr. Neil Coombs, Brandon Gagnon, Elizabeth Galati,
Prof. Oleg Gang, Ilya Gourevich, Prof. Jesse Greener, Prof. Axel Guenther, Lili Han, Dr. Anna
Klinkova, Egor Larin, Gabriella Lestari, Dr. Gabriel Menard, Maria Michaelis, Anastasiya
Muntyanu, Dr. Dmytro Nykypanchuk, Dr. Ana Querejeta-Fernández, Prof. Michael Rubinstein,
Caroline Seiler, Prof. Doug Stephan, Dr. Héloïse Thérien-Aubin, Dr. Dan Voicu, Prof. Gilbert
Walker, Dr. Huolin Xin, and Dr. Ekaterina Zhulina. I would also like to thank all my colleagues
past and present in the Kumacheva lab – thank you for all the great memories and for making the
lab such a warm and welcoming environment!
Third, I would like to thank my committee, Prof. Warren Chan, Prof. Dwight Seferos,
Prof. Gilbert Walker, and Prof. Mitchell Winnik.
Fourth, I would like to thank those people who have made an indelible impression on me
over the course of this time – I will never forget our interminable conversations (my coconut!),
Amber (amazing!), the board games (red will always be yours, Ross!), dancing the Charleston
(Random!), Greek dancing, big sister and little brother, the trolling and teasing (Joey or
Chandler? and Always the bad guy in Avalon - sorry guys!), the pre-ordering at Starbucks, the
smiles and laughter (especially your cackle, dangerous guy! Apologize to your GF for me!), the
difference between red and green (yes, I had to go there), cheating on the brown truck (Diet
Coke I’m looking at you!), red panda, Miss Eye-poker (we’ve all done it), the sauna (we’ll find
that girl and teach her a lesson!), my HS English crew, Shark in the Water, the World, the
Universe, and all that I’ve learned from you. You know who you are
Last, I would like to thank my family, especially my brother George who is my fiercest
critic and most loyal defender, my mother whose spatula I both revere and fear, and my father
who always reminds me of the impermanence of this world. Thank you for always going above
and beyond the call of duty and being my rock. As the Fairuz song goes, I love you as much as
the sea is big and the horizon is far.
v
Author Contributions
This thesis is based on key projects that have been either published, submitted or are in
preparation for peer-reviewed scientific journals. All manuscripts were written by Rachel
Choueiri (as Rachelle M. Choueiri) with critical comments and revisions by Eugenia Kumacheva
and corresponding collaborators. Contributions are provided in detail before each chapter.
vi
Publications during Ph.D. Studies
The following is a full list of publications arising from studies carried out in the preparation of
this thesis including those published, submitted, and in preparation for peer-reviewed scientific
journals. The specific contribution of Rachel Choueiri is summarized below each article listing.
Choueiri, R.M., Galati, E., Thérien-Aubin, H., Klinkova, A., Larin, E.M., Querejeta-Fernández,
A., Han, L., Xin, H.L., Gang, O., Zhulina, E., Rubinstein, M. & Kumacheva, E. Surface
Patterning of Nanoparticles with Polymer Patches. In revision.
Contribution: designed and performed experiments, imaged structures and contributed to
data analysis, co-wrote manuscript,
Choueiri, R.M.*, Galati, E.*, Klinkova, A., Thérien-Aubin, H. & Kumacheva, E. Linear
Assembly of Patchy and Non-Patchy Nanoparticles. Faraday Discuss., (2016). Accepted. DOI:
10.1039/C6FD00057F *Denotes equal contribution.
Contribution: prepared figures and co-wrote manuscript.
Klinkova, A. Therien-Aubin, H., Ahmed, A., Nykypanchuk, D., Choueiri, R.M., Gagnon, B.,
Muntyanu, A., Gang, O., Walker, G.C., & Kumacheva, E. Structural and Optical Properties of
Self-Assembled Chains of Plasmonic Nanocubes. Nano Lett. 14, 6314–6321 (2014). DOI:
10.1021/nl502746h
Contribution: synthesized nanocubes.
vii
Voicu, D., Abolhasani, M., Choueiri, R.M., Lestari, G., Seiler, C., Menard, G., Greener, J.,
Guenther, A., Stephan, D.W., & Kumacheva, E. Microfluidic Studies of CO2 Sequestration by
Frustrated Lewis Pairs. J. Am. Chem. Soc. 136, 3875–3880 (2014). DOI: 10.1021/ja411601a
Contribution: helped optimize the microfluidic system used for experiments including
device-solvent compatibility screening.
Klinkova, A.*, Choueiri, R. M.* & Kumacheva, E. Self-assembled plasmonic nanostructures.
Chem. Soc. Rev. 43, 3976–91 (2014). DOI: 10.1039/c3cs60341e *denotes equal contribution.
Contribution: co-wrote manuscript, specifically section on self-assembly, modelling of
self-assembly and contributed to applications and outlook sections.
Choueiri, R. M., Klinkova, A., Thérien-Aubin, H., Rubinstein, M. & Kumacheva, E. Structural
Transitions in Nanoparticle Assemblies Governed by Competing Nanoscale Forces. J. Am.
Chem. Soc. 135, 10262–10265 (2013). DOI: 10.1021/ja404341r
Contribution: synthesized nanoparticles, performed and imaged self-assembly
experiments, performed wetting angle measurements, spectroscopy, aided in manuscript
preparation.
Klinkova, A., Thérien-Aubin, H., Choueiri, R. M., Rubinstein, M. & Kumacheva, E. Colloidal
analogs of molecular chain stoppers. Proc. Natl. Acad. Sci. U. S. A. 110, 18775–9 (2013). DOI:
10.1073/pnas.1315381110
Contribution: synthesized nanorods.
viii
Table of Contents
Acknowledgments.......................................................................................................................... iv
Table of Contents ......................................................................................................................... viii
List of Tables ................................................................................................................................. xi
List of Figures ............................................................................................................................... xii
List of Appendices .........................................................................................................................xv
List of Abbreviations ................................................................................................................... xvi
Chapter 1 Introduction: Polymer-functionalized nanoparticles .......................................................1
1.1 Polymer-functionalized nanoparticles .................................................................................1
1.1.1 Properties of polymer-coated nanoparticles.............................................................2
1.1.2 Preparation of polymer-functionalized nanoparticles ..............................................6
1.2 Self-assembly of polymer-coated nanoparticles ..................................................................8
1.3 Patchy polymer-coated nanoparticles ................................................................................10
1.3.1 Preparation methods of patchy particles ................................................................10
1.3.2 Self-assembly of patchy nanoparticles ...................................................................14
1.3.3 Patchy nanoparticles from polymer-coated nanoparticles .....................................15
1.4 Outlook ..............................................................................................................................16
1.5 References ..........................................................................................................................17
Chapter 2 Materials and Methods ..................................................................................................22
2.1 Materials ............................................................................................................................22
2.1.1 Polymers ................................................................................................................22
2.2 Methods..............................................................................................................................24
2.2.1 Synthesis of inorganic nanoparticles .....................................................................24
2.2.2 Polymer surface segregation experiments .............................................................35
2.2.3 UV crosslinking of segregated polymer on the surface of gold NSs .....................36
ix
2.2.4 Measurement of wetting angle of polystyrene solution on gold surfaces ..............36
2.3 References ..........................................................................................................................38
Chapter 3 Structural Transitions in Nanoparticle Assemblies Governed by Competing
Nanoscale Forces ......................................................................................................................40
3.1 Introduction ........................................................................................................................40
3.2 Results and Discussion ......................................................................................................41
3.2.1 Self-assembly experiments ....................................................................................41
3.2.2 Competition of nanoscale forces ............................................................................48
3.2.3 Validation of the proposed concept .......................................................................52
3.3 Conclusions ........................................................................................................................54
3.4 References ..........................................................................................................................55
Chapter 4 Surface Patterning of Nanoparticles with Polymer Patches ..........................................59
4.1 Introduction ........................................................................................................................59
4.2 Results and Discussion ......................................................................................................63
4.2.1 Cross-linking of patchy particles ...........................................................................66
4.2.2 Tomography experiments ......................................................................................68
4.2.3 Polymer segregation control experiments ..............................................................71
4.2.4 Effect of nanosphere diameter on patch formation ................................................74
4.2.5 Effect of polymer grafting density on patch formation .........................................79
4.3 Conclusions ........................................................................................................................81
4.4 References ..........................................................................................................................83
Chapter 5 Exploring the Versatility and Applications of the Nanopatterning Method .................88
5.1 Introduction ........................................................................................................................88
5.2 Results and Discussion ......................................................................................................89
5.2.1 Polymer segregation on nanoparticles with different shapes .................................89
5.2.2 Surface segregation of different polymer ligands ..................................................98
x
5.2.3 Self-assembly of single-patch nanospheres .........................................................100
5.2.4 Self-assembly of surface-patterned nanocubes ....................................................105
5.3 Conclusions ......................................................................................................................107
5.4 References ........................................................................................................................108
Chapter 6 Conclusions and Outlook ............................................................................................109
6.1 Summary and Conclusions ..............................................................................................109
6.2 Outlook ............................................................................................................................111
Appendix 1: Derivation of calculation of nanoscale forces involved in self-assembly of PS-
coated gold NPs ......................................................................................................................114
Determination of the interfacial tension between PS-DMF/water and DMF/water phases ....114
Calculations of the change in energy following NP assembly ................................................115
References ...............................................................................................................................122
Appendix 2: Theoretical derivation of patchy nanoparticle formation ........................................123
Theoretical analysis of the state of the polymer layer on the nanosphere surface in a poor
solvent ..............................................................................................................................123
Diagram of the states of polymer molecules end-grafted to the nanosphere surface in a
poor solvent ......................................................................................................................123
Theoretical analysis of experimentally relevant polymer states on the nanosphere surface ...126
References ...............................................................................................................................133
Copyright Acknowledgments ......................................................................................................134
xi
List of Tables
Table 1. Table of molecular weight and dispersity of thiolated-polystyrene used in this work ... 23
Table 2. Solutions of thiol-terminated polystyrene used for ligand exchange ............................. 30
Table 3. Molar absorptivity of gold NSs with different diameters ............................................... 31
Table 4. Variation in polymer grafting density with polymer concentration in the solution in
ligand exchange process. .............................................................................................................. 33
Table 5. Effect of the addition of NaCl and THF on nanoparticle assembly ................................ 53
xii
List of Figures
Figure 1.1.1 Schematic representation of a polymer-coated nanoparticle and their applications
explored in this thesis...................................................................................................................... 2
Figure 1.1.2 Schematic representation of polymers grafted to a surface. ....................................... 3
Figure 1.1.3: Effect of polymer grafting density on the interaction between nanoparticles
functionalized with polymer brushes. ............................................................................................. 4
Figure 1.1.4 Schematic of brush regimes........................................................................................ 5
Figure 1.3.1 Common methods for the preparation patchy nanoparticles. ................................... 11
Figure 1.3.2 Strategies towards patchy particles using diblock copolymers and mixed polymer
brushes grafted onto nanoparticle surfaces. .................................................................................. 13
Figure 1.3.3 Self-assembly of patchy particles. ............................................................................ 15
Figure 2.2.1. Representative thermal gravimetric analysis traces. ............................................... 32
Figure 3.2.1 Transmission electron microscopy (TEM) images of the representative NP species
observed in NP solution in DMF-water mixture........................................................................... 43
Figure 3.2.2 Phase-like diagram of the self-assembled structures of gold NPs, plotted in the Mn−
Cw space with tSA= 1 h. ................................................................................................................. 44
Figure 3.2.3 Testing the reversibility of self-assembled chains and globules of gold
nanoparticles. ................................................................................................................................ 45
Figure 3.2.4 Characterization of globules and the interparticle spacing of nanoparticles
functionalized with PS. ................................................................................................................. 47
Figure 3.2.5 Schematics of nanoparticle assembly and angle-dependence of nanoscale forces. . 51
Figure 3.2.6 Fraction of population of globules and chains formed by gold NPs stabilized with
PS-50K at Cw of 5, 10, and 15 vol. % with tSA=1h. ...................................................................... 52
Figure 4.1.1 Polymer segregation on the NP surface. ................................................................... 62
Figure 4.2.1. Extinction spectra of gold NSs. ............................................................................... 64
Figure 4.2.2. TEM images of gold NSs stabilized with PS-50K. ................................................. 64
Figure 4.2.3. TEM images of gold NSs stabilized with PS-30K. ................................................. 65
Figure 4.2.4 Segregation of the PS-co-PI copolymer on the surface of gold NSs. ....................... 68
xiii
Figure 4.2.5 Reconstructed 3D images of a 60 nm-diameter patchy NS functionalized with PS-
50K. ............................................................................................................................................... 70
Figure 4.2.6 Polymer-capped gold NSs in a good solvent. ........................................................... 72
Figure 4.2.7 Polymer-capped gold NSs in the DMF/water mixture at Cw =10 vol. %. ................ 73
Figure 4.2.8 TEM images of polystyrene-stabilized gold NSs (3.0 nM concentration) in the
DMF/water mixture. ..................................................................................................................... 74
Figure 4.2.9. Effect of NS curvature to polymer size ratio on surface patch formation. .............. 75
Figure 4.2.10. Polymer segregation on the surface of gold NSs................................................... 76
Figure 4.2.11. Polymer segregation on the surface of gold NSs................................................... 77
Figure 4.2.12. Polymer segregation on the surface of gold NSs................................................... 77
Figure 4.2.13. Effect of grafting density on polymer segregation on the NS surface. ................. 78
Figure 4.2.14. Effect of grafting density on polymer segregation on the surface of NSs. ............ 78
Figure 4.2.15. Structural transitions in the polymer layer on the surface of gold NSs. ................ 79
Figure 5.2.1. Electron microscopy images of gold NRs. .............................................................. 90
Figure 5.2.2. TEM images of gold NDs........................................................................................ 91
Figure 5.2.3. TEM images of silver NCs. ..................................................................................... 91
Figure 5.2.4. TEM images of gold NCs. ....................................................................................... 92
Figure 5.2.5. Generality of polymer patterning of NP surface. .................................................... 93
Figure 5.2.6. Polymer segregation on the surface of gold NRs. ................................................... 95
Figure 5.2.7. Polymer segregation on the surface of gold NDs. ................................................... 96
Figure 5.2.8. Polymer segregation on the surface of silver NCs. ................................................. 96
Figure 5.2.9. Polymer segregation on the surface of gold NCs. ................................................... 97
Figure 5.2.10. Polymer segregation on the surface of gold NCs in the THF-water mixture. ....... 98
Figure 5.2.11. Surface segregation of polymer on the surface of gold NSs. ................................ 99
Figure 5.2.12. Self-assembly of single-patch gold nanospheres. ................................................ 101
Figure 5.2.13. Dark-field mode SEM image of self-assembled chains of single-patch gold
nanospheres following their 45 min sonication. ......................................................................... 102
xiv
Figure 5.2.14. Self-assembly of patchy NSs in the presence of excess polymer. ....................... 104
Figure 5.2.15. Self-assembly of surface-patterned and non-patterned nanocubes. .................... 106
Figure 5.2.16. Self-assembly of patchy NCs functionalized with PS-50K in an open
checkerboard structure. ............................................................................................................... 107
xv
List of Appendices
Appendix 1: Derivation of calculation of nanoscale forces involved in self-assembly of PS-
coated gold NPs .......................................................................................................................... 114
Appendix 2: Theoretical derivation of patchy nanoparticle formation ....................................... 123
xvi
List of Abbreviations
CPC cetylpyridinium chloride
CTAB cetyltrimethylammonium bromide
Cw concentration of water in volume percent
DMF N,N-dimethylformamide
Mn number average molar mass
NC nanocube
ND nanorod with a dumbbell shape (nanodumbbell)
NP nanoparticle
NR spherocylindrical nanorod
NS nanosphere
PS polystyrene
PS-co-PI polystyrene-co-polyisoprene
PVK poly(N-vinyl carbazole)
P4VP poly(4-vinyl pyridine)
THF tetrahydrofuran
1
Chapter 1
Introduction: Polymer-functionalized nanoparticles
Portions of this chapter were adapted from: Klinkova, A.*, Choueiri, R. M.* & Kumacheva, E.
Self-assembled plasmonic nanostructures. Chem. Soc. Rev. 43, 3976–91 (2014). *denotes equal
contribution DOI: 10.1039/c3cs60341e
Contribution: R. M. Choueiri wrote the section on self-assembly and modelling of self-assembly
and co-wrote the section on applications with minor contributions to the introduction and
conclusion.
1.1 Polymer-functionalized nanoparticles
Polymer functionalized nanoparticles (Figure 1.1.1 left side) have extensive applications
in medicine1, waste water remediation2,3, and sensing4,5 with specific applications such as
imaging and diagnostic agents in point-of-care devices. These materials combine the
functionality of the nanoparticle core which may be, for example, composed of a plasmonic or
fluorescent material with the unique properties of polymers – conferring colloidal stability,
cross-linkability or stimuli-responsiveness and reversible association interactions6. Due to the
attractiveness of such hybrid materials, significant research effort has been invested in the
preparation and application of these materials. This thesis deals with two emerging applications
of polymer-functionalized nanoparticles namely, their self-assembly7 and their use in the
2
generation of patchy nanoparticles8 (Figure 1.1.1 right side).
Figure 1.1.1 Schematic representation of a polymer-coated nanoparticle and their
applications explored in this thesis. Figure adapted from reference 8 with permission.
1.1.1 Properties of polymer-coated nanoparticles
Before we can discuss the applications of polymer-coated nanoparticles it is important to
understand their characteristics and develop a vocabulary to describe their properties. For the
purposes of this thesis we will consider polymer-coated nanoparticles to mean systems
comprising of a solid nanoparticle core composed of a metal or semi-conductor which is
protected by polymers that are end-tethered to the surface. We assume that for the most part
there is only one point of attachment of the polymer to the particle, resulting in a polymer brush
decorating the surface of the nanoparticle (Figure 1.1.1 left side, Figure 1.1.2).
3
One way of characterizing the resultant brush formed is in determining the average
number of these polymer attachments or grafts which exist per unit surface area of the
nanoparticle, commonly referred to as the grafting density, σ. Experimentally, the grafting
density may be determined by a variety of techniques including thermogravimetric analysis9,
elemental analysis, inductively couple plasma atomic emission spectroscopy (ICP-AES) and
NMR10. The grafting density will in part determine the behaviour of the brush with respect to
self-assembly and colloidal interactions with other species as well as surface charge and
hydrodynamic radius11.
Figure 1.1.2 Schematic representation of polymers grafted to a surface. The grafted
polymers have a characteristic brush height, h, and distance between polymer grafts, D.
Reproduced with permission from Wiley (reference 11).
In particular, the brush regime, which is a reflection of the conformation of the polymer
chains and is determined by the polymer grafting density, plays an important role in particle-
particle interactions (Figure 1.1.3) which impacts the self-assembly behaviour of nanoparticles12.
At low grafting densities, polymer self-association is favourable, and nanoparticles may self-
associate due to a lack of steric penalty. As the grafting density increases, steric hindrance
prevents the association of nanoparticles, effectively hindering self-assembly.
4
Figure 1.1.3: Effect of polymer grafting density on the interaction between nanoparticles
functionalized with polymer brushes. (a) Attraction at low grafting density and (b) steric
repulsion at moderate grafting density. Reproduced with permission from Wiley (reference 11).
Depending on the grafting density, different brush regimes may be accessed. These brush
regimes are determined by the available space accessible to the polymer grafts, D, as well as the
preference of interaction of the polymer to the surface or to the solvent. The brush regime is
correlated to the height, h, of the polymer layer (Figure 1.1.2) as well as the stretching energy of
the tethered polymers13. A schematic of how brush regimes change with increasing grafting
density is given in Figure 1.1.4.
5
Figure 1.1.4 Schematic of brush regimes. Variation of polymer brush height with polymer
grafting density leading to different brush regimes. Each polymer graft becomes increasingly
constrained as the grafting density increases. Reproduced with permission from MDPI (reference
13).
As illustrated in Figure 1.1.4, the brush regime dictates the conformation of the polymer
tethered to the surface of the nanoparticle. In the mushroom regime, the tethered polymers
experience no impingement from adjacent polymer grafts and the radius of each graft is equal to
the radius of gyration, Rg. As the grafting density increases to the moderate-density regime,
polymer grafts become constrained and must stretch. The distance between grafts, D, is now less
than twice the radius of gyration. At a maximal grafting density (the high density regime) the
polymer grafts reach their terminal brush height and can no longer stretch.
In addition to the effect of grafting density on brush height, the polymer conformation
also changes with response to a stimulus. For instance, the polymer brush height, h, is
proportional to the solvent quality as h ∝ Nσυ, where N is the degree of polymerization, and υ is
6
an exponent typically ranging from 0 to 1 that accounts for solvent quality. The value of υ is
typically 1/3 in good solvents, 1/2 in Θ-solvents and higher for poor solvents14.
Grafting density is controllable within a certain range depending on the preparation
method used to prepare the polymer-functionalized nanoparticles and the affinity for the polymer
end-group for the nanoparticle substrate material.
1.1.2 Preparation of polymer-functionalized nanoparticles
There are two main methods to functionalize pre-formed nanoparticles with polymer: the
“grafting-from” and “grafting-to” methods. “Grafting from” involves functionalizing the
nanoparticle with a polymer initiator and then growing the polymer chain from the surface of the
nanoparticle. Surface-initiated polymerizations have reached a state of the art and anionic,
cationic, surface initiated atom transfer radical polymerization and surface initiated reverse
addition-fragmentation chain-transfer polymerizations have all been achieved from the surface of
silica and other inorganic particles15, 16. Control of the molecular weight is possible however, its
characterization is complicated by the fact that the polymers are tethered to the surface.
Destructive characterization methods such as dissolution of the nanoparticle scaffold to obtain
the polymer to characterize have been developed. A non-destructive method involves a
sacrificial initiator which is left untethered to the nanoparticle and polymerized alongside the
tethered initiators. Then, once polymerization has been stopped, the sacrificial polymer is
collected and characterized with the assumption that its molecular weight does not differ
significantly from that of the tethered polymer.
Another complication involves the dispersity of the grown polymers. In cases where the
desired grafting density is high, the adjacent growing chains may begin to impinge—resulting in
7
the termination of one of the neighbouring chains. This leads to a more variable molecular
weight11.
By contrast, “grafting to” involves the association of a pre-formed polymer chain to the
surface of the nanoparticle15. This is achieved most commonly in a ligand exchange experiment,
whereby the pre-existing surface ligands are replaced by the added polymer due to a higher
affinity of the polymer binding group to the surface of the nanoparticle. With the grafting-to
method there is less uncertainty in the molecular weight or dispersity of the polymers as they are
pre-formed however, the ligand exchange step may be complicated especially for long polymer
grafts with one anchoring group.
Both methods have specific advantages and disadvantages with regards to the grafting
density of ligands achievable, the types of polymer ligands that can be attained as well as
specific challenges in the determination of grafting density16. Grafting-from methods typically
have higher grafting densities accessible than grafting-to methods. The maximum grafting
density achievable for grafting-to methods varies inversely with polymer molecular weight.
Thus, for applications requiring true brushes, the grafting-to method is only suitable for lower
molecular weight polymers.
The control of grafting density in grafting-from methods is commonly achieved by
functionalizing the nanoparticle surface with a mix of initiator and dummy initiator (which is
structurally similar) in the desired ratio. In the case of grafting-to, the grafting density may be
controlled by adjusting the amount of polymer ligand added to the polymer solution as well as
adjusting the time allowed for polymer association before removal of free polymer via
centrifugation, for example.
8
Polymer brushes may be made from any combination of homopolymers, block
copolymers, and short ligands in varying proportions and with differing molecular weights. The
nature of the polymer and the composition of the mixture dictates the functionality and
morphology of the resultant brush affecting the colloidal stability, self-assembly and stimuli-
responsive behavior of the nanoparticle.
1.2 Self-assembly of polymer-coated nanoparticles
Self-assembly, the process by which building blocks such as nanoparticles self-organize
into ordered structures, has attracted much interest as a potential tool to generate arrays of
nanostructures for integration into functional devices. The study of self-assembled structures is
fundamental to the understanding of structure-property relationships – specifically with respect
to the coupling of adjacent nanoparticles in an ensemble. For example, plasmonic nanoparticles
may organize into small clusters or lattices comprising thousands of nanoparticles offering
another degree of freedom in controlling and tuning their optical properties by coupling surface
plasmons of adjacent nanoparticles and enhancing the electric field in interparticle gaps (so-
called, “hot spots”). The enhancement of electric field strongly influences the chemical and
optical properties of molecules or other nanoparticles, e.g., quantum dots, placed in the hot spots.
Control over plasmonic properties of self-assembled nanostructures can be achieved by changing
the number of nanoparticles in the cluster, by varying the directionality of nanoparticle
organization, and by fine- tuning interparticle distance, in addition to changing the composition,
size and shape of individual nanoparticles. The array of variables at play in the self-assembly
process and the interplay of these variables and the resultant properties of the achieved structure
make this field complex and highly interdisciplinary.
9
Building on previous work in our group and with a desire to further explore the properties
of polymer-coated nanoparticles we decided to study the self-assembly of polymer-
functionalized nanoparticles in solution. The self-assembly of polymer-coated nanoparticles in
solution is dependent on nanoscale forces, polymer interactions, and colloid theory all of which
can be changed by varying solution and building block parameters. For example, self-assembled
structures achieved with polymer-coated nanoparticles include vesicles made from amphiphilic
block-copolymer-functionalized gold nanoparticles assembled in selective solvent mixtures17, 18,
as well as chains assembled from gold nanoparticles functionalized with a pH responsive
polymer19. The photothermal responsiveness of the particle-polymer hybrids allowed for the
control of payload release in the case of self-assembled vesicles, while the pH responsiveness of
the polymer in the case of self-assembled gold nanoparticle chains allowed for the precise
control of the chain aggregation length by varying the medium pH.
Additionally, regiospecifically functionalized gold nanorods having polymer ligands at
the tips of the nanorod assembled into chains by solvophobic attraction of the polystyrene
pompoms in a poor solvent20. Using the analogy of molecular polymers, these chains of nanorods
could be considered as nanopolymers – opening the door for new strategies in self-assembly
adapted from the molecular world. For instance, control of the nanorod aggregation number was
achieved in a number of ways, including permanent cross-linking of the assembled structures21,
the use of patchy heterostructured particles as monofunctional monomers behaving as chain-
stoppers to zip the ends of nanorod chains22 and lipid encapsulation of nanorod chains23. Control
of aggregation number is highly important with respect to maximizing the surface enhanced
Raman scattering (SERS) of targeted analytes. Additionally, control of inter-nanorod bond
angles and lengths was achieved through cross-linking of the physically associated polymer
10
ligands. All examples show the versatility of polymer-nanoparticle composite building blocks for
self-assembly.
1.3 Patchy polymer-coated nanoparticles
Patchy nanoparticles have many applications including multifunctional probes, colloidal
surfactants and building blocks for unique self-assembly modes24. Due to the potential for
complex structures25 and bonding modalities analogous to those of atoms26 a great deal of
research has been invested in the formation of patchy nanoparticles and studying their
behaviour27.
1.3.1 Preparation methods of patchy particles
Patchy micro- and nano- particles can be formed by a variety of methods including but
not limited to: masking28, microfluidics29, ligand assembly at the interface of two liquids30, phase
segregation of block copolymer ligands31, and inorganic heterostructure formation32. Most
syntheses produce binary or Janus nanoparticles however, multi-patch and raspberry structures
have been obtained as well33. A schematic representation of some of the common methods of
two-patch or Janus nanoparticle preparation is presented in Figure 1.3.1.
11
Figure 1.3.1 Common methods for the preparation patchy nanoparticles. Reprinted with
permission from Elsevier (reference 27).
Masking involves embedding isotropically functionalized particles in a resin or matrix
such that only part of the particle is exposed. Then, surface modification is carried out on the
exposed portion of the particle. After functionalization, the matrix is dissolved and the free
particles are anisotropically functionalized. Microfluidic methods include the emulsification of
droplets composed of immiscible monomers. Once the monomers are polymerized by UV light,
the interfacial boundary between the two polymer phases is preserved in the resultant
microparticle. By adjusting monomer flow rates, the volume fraction of each phase was
controlled34. This method allowed for the preparation of binary and ternary particles.
12
Exploiting the interface of two immiscible solutions of ligands, it is possible to take an
inorganic nanoparticle and simultaneously functionalize both halves with chemically distinct
ligands as the particles will naturally assemble at the interface to reduce the interfacial tension.
Once assembled at the interface, the ligand exchange occurs resulting in an amphiphilic Janus
nanoparticle.
Stabilizing nanoparticles with block copolymers or mixed polymer brushes allows for the
formation of patchy nanoparticles using selective solvents. The patch size can be controlled by
tuning the sizes of each block or the grafting density of each homopolymer. It is predicted that
multi-patch, multi-“hole” and raspberry morphologies can be achieved using both grafted
diblock copolymers on a spherical core35 (Figure 1.3.2a) and mixed polymer brushes grafted to a
spherical core36 (Figure 1.3.2b-e). Qualitatively similar structures have been achieved using
diblock copolymers assembled in an emulsion37 (Figure 1.3.2f).
13
Figure 1.3.2 Strategies towards patchy particles using diblock copolymers and mixed
polymer brushes grafted onto nanoparticle surfaces. (a) Change in surface morphology for a
diblock copolymer grafted to a spherical nanoparticle with changing block fraction, f. (b-e)
Change in surface morphology for a mixed polymer brush grafted onto a spherical nanoparticle
(red core) with increasing grafting density fraction σB/σA from left to right. Blue represents
polymer A, and green represents polymer B. (f) Patchy block-copolymer colloids formed by
confined assembly of micelles in an emulsion. The number of patches could be controlled by
changing the size of the confining emulsion droplets. Reprinted with permission from references
35, 36, and 37.
Another strategy is to use untethered block copolymers and subject them to a selective
solvent31. The block copolymer forms a micelle on the surface of the nanoparticle to reduce the
14
interfacial area. If a strongly-binding small molecule ligand is introduced, the micelle can be
constrained to one part of the nanoparticle, creating so-called eccentric micelles. The eccentricity
of the micelle can be controlled by the amount of small molecule ligand added.
1.3.2 Self-assembly of patchy nanoparticles
The self-assembly of patchy nanoparticles has been explored for patchy microparticles24
as well as nanoparticles31. The possibility to tune patch position, size and number as well as to
modify the shape of the building block and the nature of patch-patch interactions opens up many
avenues to potentially useful self-assembled structures (Figure 1.3.3a). In the case of patchy
microparticles, extended 2D assemblies such as kagome lattices made from ternary striped
patchy particles25 (Figure 1.3.3b), 1D assemblies of chains and molecule-mimicking assemblies
from multi-patch particles26 (Figure 1.3.3) have been established.
15
Figure 1.3.3 Self-assembly of patchy particles. (a) Schematic of tunable variables in patchy
particle formation. (b) Left: Fluorescence image of colloidal kagome lattice formed by striped
particle assembly. Inset depicts fast Fourier transform of main image. Scale bar is 4 µm. Top
right: enlarged view of region in white dashed border. Bottom right: schematic representation of
patchy colloid orientation in assembly (c) Patchy colloidal analogues of atoms showing
directional bonding modalities. Leftmost column depicts bright field images, middle column
confocal fluorescent microscopy and rightmost column contains schematic representations of the
assemblies. Scale bars are 2 µm. Reprinted with permission from references 24, 25 and 26.
1.3.3 Patchy nanoparticles from polymer-coated nanoparticles
The generation of patchy nanoparticles from the phase separation of surface ligands has
been established for mixtures of small molecule ligands38, mixtures of polymers and small
molecules39, mixtures of polymer ligands40, and block copolymers41. Polymer ligands offer an
16
attractive modality for the generation of patchy particles given their higher propensity to phase
segregate relative to small molecule ligands. Characterization of patchy nanoparticles generated
from the phase segregation of surface-bound ligands remains a challenge and relies heavily on
indirect methods of characterization such as self-assembly or the growth of an inorganic species
on a specific domain39.
1.4 Outlook
Polymer-functionalized nanoparticles have promise as building blocks for self-assembly,
biomedical agents and precursors to patchy nanoparticles. Central to this utility is the
composition and nature of the polymer brush on the surface of the nanoparticle. Surface
modification of nanoparticles with polymers has reached a state of the art, allowing for control
and measurement of grafting density, graft molecular weight and graft phase segregation or
placement on the surface of the nanoparticle. This control will allow for the exploration of the
integration of polymer-functionalized nanoparticles in new applications and devices.
Central to this integration in devices is the ability to control the assembly of these
building blocks. Polymer-functionalized nanoparticles have a demonstrated capability to
assemble in novel and useful ways, including as responsive cargo-carriers, multifunctional
probes and sensors. The unique properties of polymers make possible previously inaccessible
self-assembly modes, including large scale vesicles and cylindrical micelles. Assemblies of
polymer-coated nanoparticles may be reversible upon a change in solution conditions or may be
permanently fixed via cross-linking. The extent of assembly and aggregation number may be
controlled by careful tuning of self-assembly time, and nanoscale forces such as electrostatic
repulsion.
17
While polymer brushes on nanoparticles have wide-ranging capabilities and applications
it is clear that their complex interactions with species in solution and the solvents themselves can
lead to complications in different media including in vivo. This is why an understanding of
polymer behaviour when tethered to a surface is important in predicting nanoparticle fate and
interactions and requires further exploration.
1.5 References
1. Mout, R., Moyano, D. F., Rana, S. & Rotello, V. M. Surface functionalization of nanoparticles
for nanomedicine. Chem. Soc. Rev. 41, 2539 (2012). DOI: 10.1039/c2cs15294k
2. Phenrat, T. & Lowry, G. V. in Micro and Nano Technologies (eds. Sustich, R., Duncan, J. &
Savage, N. B. T.-N. A. for C. W. (Second E.) 473–490 (William Andrew Publishing, 2014).
DOI: 10.1016/B978-1-4557-3116-9.00030-5
3. Farrukh, A. et al. Design of Polymer-Brush-Grafted Magnetic Nanoparticles for Highly
Efficient Water Remediation. ACS Appl. Mater. Interfaces 5, 3784–3793 (2013). DOI:
10.1021/am400427n
4. Hezinger, A. F. E., Teßmar, J. & Göpferich, A. Polymer coating of quantum dots – A
powerful tool toward diagnostics and sensorics. Eur. J. Pharm. Biopharm. 68, 138–152 (2008).
DOI: 10.1016/j.ejpb.2007.05.013
5. Zhang, F. et al. Polymer-Coated Nanoparticles: A Universal Tool for Biolabelling
Experiments. Small 7, 3113–3127 (2011). DOI: 10.1002/smll.201100608
6. Moffitt, M. G. Self-Assembly of Polymer Brush-Functionalized Inorganic Nanoparticles:
From Hairy Balls to Smart Molecular Mimics. J. Phys. Chem. Lett. 4, 3654–3666 (2013). DOI:
10.1021/jz401814s
18
7. Choueiri, R. M., Klinkova, A., Thérien-Aubin, H., Rubinstein, M. & Kumacheva, E. Structural
Transitions in Nanoparticle Assemblies Governed by Competing Nanoscale Forces. J. Am.
Chem. Soc. 135, 10262–10265 (2013). DOI: 10.1021/ja404341r
8. Choueiri, R.M., Galati, E., Thérien-Aubin, H., Klinkova, A., Larin, E.M., Querejeta-
Fernández, A., Han, L., Xin, H.L., Gang, O., Zhulina, E., Rubinstein, M. & Kumacheva, E.
Surface Patterning of Nanoparticles with Polymer Patches. Submitted.
9. Benoit, D. N. et al. Measuring the Grafting Density of Nanoparticles in Solution by Analytical
Ultracentrifugation and Total Organic Carbon Analysis. Anal. Chem. 84, 121009152814008
(2012). DOI: 10.1021/ac301980a
10. Tong, L. et al. Quantification of Surface Ligands on NaYF 4 Nanoparticles by Three
Independent Analytical Techniques. Chem. Mater. 27, 4899–4910 (2015). DOI:
10.1021/acs.chemmater.5b02190
11. Brittain, W. J. & Minko, S. A structural definition of polymer brushes. J. Polym. Sci. Part A
Polym. Chem. 45, 3505–3512 (2007). DOI: 10.1002/pola.22180
12. Nikolic, M. S. et al. Micelle and Vesicle Formation of Amphiphilic Nanoparticles. Angew.
Chemie Int. Ed. 48, 2752–2754 (2009). DOI: 10.1002/anie.200805158
13. Kim, M., Schmitt, S., Choi, J., Krutty, J. & Gopalan, P. From Self-Assembled Monolayers to
Coatings: Advances in the Synthesis and Nanobio Applications of Polymer Brushes. Polymers
(Basel). 7, 1346–1378 (2015). DOI: 10.3390/polym7071346
14. Moh, L. C. H., Losego, M. D. & Braun, P. V. Solvent Quality Effects on Scaling Behavior of
Poly(methyl methacrylate) Brushes in the Moderate- and High-Density Regimes. Langmuir 27,
3698–3702 (2011). DOI: 10.1021/la2002139
15. Advincula, R. C. Surface Initiated Polymerization from Nanoparticle Surfaces. J. Dispers.
Sci. Technol. 24, 343–361 (2003). DOI: 10.1081/dis-120021794
16. Hui, C. M. et al. Surface-Initiated Polymerization as an Enabling Tool for Multifunctional
(Nano-)Engineered Hybrid Materials. Chem. Mater. 26, 745–762 (2014). DOI:
10.1021/cm4023634
19
17. He, J., Liu, Y., Babu, T., Wei, Z. & Nie, Z. Self-Assembly of Inorganic Nanoparticle
Vesicles and Tubules Driven by Tethered Linear Block Copolymers. J. Am. Chem. Soc. 134,
11342–11345 (2012). DOI: 10.1021/ja3032295
18. He, J. et al. Hydrodynamically driven self-assembly of giant vesicles of metal nanoparticles
for remote-controlled release. Angew. Chem. Int. Ed. Engl. 52, 2463–8 (2013). DOI:
10.1002/anie.201208425
19. Xia, H., Su, G. & Wang, D. Size-dependent electrostatic chain growth of pH-sensitive hairy
nanoparticles. Angew. Chem. Int. Ed. Engl. 52, 3726–30 (2013). DOI: 10.1002/anie.201209304
20. Liu, K. et al. Step-growth polymerization of inorganic nanoparticles. Science 329, 197–200
(2010). DOI: 10.1126/science.1189457
21. Lukach, A., Liu, K., Therien-Aubin, H. & Kumacheva, E. Controlling the degree of
polymerization, bond lengths, and bond angles of plasmonic polymers. J. Am. Chem. Soc. 134,
18853–9 (2012). DOI: 10.1021/ja309475e
22. Klinkova, A., Thérien-Aubin, H., Choueiri, R. M., Rubinstein, M. & Kumacheva, E.
Colloidal analogs of molecular chain stoppers. Proc. Natl. Acad. Sci. U. S. A. 110, 18775–9
(2013). DOI: 10.1073/pnas.1315381110
23. Stewart, A. F. et al. Rational design for the controlled aggregation of gold nanorods via
phospholipid encapsulation for enhanced Raman scattering. ACS Nano 8, 5462–5467 (2014).
DOI: 10.1021/nn4044589
24. Chen, Q., Yan, J., Zhang, J., Bae, S. C. & Granick, S. Janus and Multiblock Colloidal
Particles. Langmuir 28, 13555–13561 (2012). DOI: 10.1021/la302226w
25. Chen, Q., Bae, S. C. & Granick, S. Directed self-assembly of a colloidal kagome lattice.
Nature 469, 381–384 (2011). DOI: 10.1038/nature09713
26. Wang, Y. et al. Colloids with valence and specific directional bonding. Nature 490, 51–55
(2012). DOI: 10.1038/nature11564
27. Lattuada, M. & Hatton, T. A. Synthesis, properties and applications of Janus nanoparticles.
Nano Today 6, 286–308 (2011). DOI: 10.1016/j.nantod.2011.04.008
20
28. Jiang, S. et al. Janus Particle Synthesis and Assembly. Adv. Mater. 22, 1060–1071 (2010).
DOI: 10.1002/adma.200904094
29. Roh, K., Martin, D. C. & Lahann, J. Biphasic Janus particles with nanoscale anisotropy. Nat.
Mater. 4, 759–763 (2005). DOI: 10.1038/nmat1486
30. Andala, D. M., Shin, S. H. R., Lee, H. Y. & Bishop, K. J. M. Templated synthesis of
amphiphilic nanoparticles at the liquid-liquid interface. ACS Nano 6, 1044–1050 (2012). DOI:
10.1021/nn202556b
31. Chen, T., Yang, M., Wang, X., Li, H. T. & Chen, H. Controlled assembly of eccentrically
encapsulated gold nanoparticles. J. Am. Chem. Soc. 130, 11858–11859 (2008). DOI:
10.1021/ja8040288
32. Yu, H. et al. Dumbbell-like Bifunctional Au−Fe3O4 Nanoparticles. Nano Lett. 5, 379–382
(2005). DOI: 10.1021/nl047955q
33. Walther, A. & Müller, A. H. E. Janus Particles: Synthesis, Self-Assembly, Physical
Properties, and Applications. Chem. Rev. 113, 5194–5261 (2013). DOI: 10.1021/cr300089t
34. Nie, Z., Li, W., Seo, M., Xu, S. & Kumacheva, E. Janus and Ternary Particles Generated by
Microfluidic Synthesis: Design, Synthesis, and Self-Assembly. J. Am. Chem. Soc. 128, 9408–
9412 (2006). DOI: 10.1021/ja060882n
35. Vorselaars, B., Kim, J. U., Chantawansri, T. L., Fredrickson, G. H. & Matsen, M. W. Self-
consistent field theory for diblock copolymers grafted to a sphere. Soft Matter 7, 5128 (2011).
DOI: 10.1039/c0sm01242d
36. Wang, Y. et al. Mixed homopolymer brushes grafted onto a nanosphere. J. Chem. Phys. 134,
134903 (2011). DOI: 10.1063/1.3575180
37. Ku, K. H., Kim, Y., Yi, G.-R., Jung, Y. S. & Kim, B. J. Soft Patchy Particles of Block
Copolymers from Interface-Engineered Emulsions. ACS Nano (2015). DOI:
10.1021/acsnano.5b05058
21
38. Jackson, A. M., Myerson, J. W. & Stellacci, F. Spontaneous assembly of subnanometre-
ordered domains in the ligand shell of monolayer-protected nanoparticles. Nat. Mater. 3, 330–
336 (2004). DOI: 10.1038/nmat1116
39. Chen, T., Chen, G., Xing, S., Wu, T. & Chen, H. Scalable routes to janus Au-SiO2 and
ternary Ag-Au-SiO2 nanoparticles. Chem. Mater. 22, 3826–3828 (2010). DOI:
10.1021/cm101155v
40. Zhou, T., Dong, B., Qi, H., Mei, S. & Li, C. Y. Janus hybrid hairy nanoparticles. J. Polym.
Sci. Part B Polym. Phys. 52, 1620–1640 (2014). DOI: 10.1002/polb.23611
41. Zubarev, E. R., Xu, J., Sayyad, A. & Gibson, J. D. Amphiphilicity-driven organization of
nanoparticles into discrete assemblies. J. Am. Chem. Soc. 128, 15098–9 (2006). DOI:
10.1021/ja066708g
22
Chapter 2
Materials and Methods
Portions of this chapter were adapted from:
Choueiri, R.M., Galati, E., Thérien-Aubin, H., Klinkova, A., Larin, E.M., Querejeta-Fernández,
A., Han, L., Xin, H.L., Gang, O., Zhulina, E., Rubinstein, M. & Kumacheva, E. Surface
Patterning of Nanoparticles with Polymer Patches. Submitted.
Choueiri, R. M., Klinkova, A., Thérien-Aubin, H., Rubinstein, M. & Kumacheva, E. Structural
Transitions in Nanoparticle Assemblies Governed by Competing Nanoscale Forces. J. Am.
Chem. Soc. 135, 10262–10265 (2013). DOI: 10.1021/ja404341r
2.1 Materials
All materials were purchased from commercial suppliers and used without further purification
unless otherwise stated.
2.1.1 Polymers
2.1.1.1 Thiol-terminated polystyrene
Thiol-terminated polystyrene (PS) synthesized by anionic polymerization was purchased from
Polymer Source, Inc. (Dorval, Canada). PS samples with five molecular weights (Mn = 5000, 12
000, 20 000, 29 000 and 50 000 g/mol referred to in the text as PS-5K, PS-12K, PS-20K, PS-
30K, and PS-50K, respectively) were used (Table 1).
23
Table 1. Table of molecular weight and dispersity of thiolated-polystyrene used in this
work
Abbreviated name Mn (g/mol)* Dispersity*, Đ
PS-5K 5 000 1.4
PS-12K 12 000 1.09
PS-20K 20 000 1.07
PS-30K 29 000 1.07
PS-50K 50 000 1.06
*The molecular weight and dispersity were provided by the supplier
2.1.1.2 Thiol-terminated random copolymer poly(styrene-co-isoprene).
Thiol-terminated random copolymer poly(styrene-co-isoprene) (PS-co-PI) with a
molecular weight Mn= 53,000 g/mol and a dispersity of 1.16 was purchased from Polymer
Source (Dorval, Canada). The copolymer was prepared by the anionic copolymerization of
styrene and isoprene. The polymerization was terminated by the addition of propylene sulfide.
2.1.1.3 Thiol-terminated poly(4-vinyl pyridine)
Thiol-terminated poly(4-vinyl pyridine) was prepared by Dr. Héloïse Thérien-Aubin via
anionic polymerization1. The polymerization was carried out in a tetrahydrofuran (THF)/
hexamethylphosphoric triamide (HMPT) mixture (186 mL of THF and 4 mL of HMPT) and
initiated with 1,1-diphenyl-3-methylpentyllithium prepared by the reaction of sec-butyl lithium
(9.75 mg) with diphenylethylene (27.4 mg). The monomer 4-vinyl pyridine (6 g) was added to
the solution of initiator at -78 °C. Polymerization took place for 2 h at room temperature and was
24
terminated by adding ethylene sulphide (10 mg). The molecular weight of P4VP was Mn= 40000
g/mol with a PDI of 1.05.
2.1.1.4 Thiol-terminated poly(N-vinylcarbazole)
Thiol-terminated poly(N-vinylcarbazole) with Mn =19200 g/mol and a dispersity of 1.34
was received from the group of Prof. Krzysztof Matyjaszewski (Carnegie Mellon University).
The polymer was synthesized by RAFT polymerization of N-vinylcarbazole in the presence of S-
(2-Ethyl propionate) O-ethyl xanthate2,3 and subsequent aminolysis of the xanthate chain-end4.
2.1.1.5 Polystyrene.
A polystyrene standard with a molecular weight Mn= 50000 g/mol and a dispersity of
1.06 was supplied by Alfa Aesar Corporation.
2.2 Methods
2.2.1 Synthesis of inorganic nanoparticles
2.2.1.1 Synthesis of gold nanospheres
Gold nanoparticles with a spherical shape (nanospheres, NSs) with diameter of 19 ± 1 nm
were synthesized using a two-step seed-mediated procedure reported by the Murphy group5. A
seed solution was prepared by reducing HAuCl4 (10 mL, 5×10-4 M) in 10 mL of aqueous citrate
solution (5 × 10-4 M) via the addition of ice-cold solution of NaBH4 in water (0.6 mL, 0.1 M)
under stirring. The stirring was stopped 10 min after the addition of NaBH4 solution, and the
resultant solution was stored at room temperature for 2-3 h. A second solution, denoted as
"Solution A" was prepared by dissolving HAuCl4 (2.5×10-3 M) and cetyltrimethylammonium
bromide (CTAB) (0.08 M) in water. To 9 mL of Solution A, 0.5 mL of 0.1 M aqueous ascorbic
acid was added. Immediately after that, 10 mL of the aged seed solution was added to the above
25
mixture under vigorous stirring. The resultant solution (Solution B) was stirred for 10 min and
used 30 min after its preparation. Another solution, denoted as "Solution C", was prepared by
dissolving HAuCl4 (2.5 ×10-3 M) and CTAB (0.08 M) in water. To 9 mL of Solution C, 0.5 mL
of 0.1 M aqueous ascorbic acid was added. Following this step, 9 mL of Solution B was added
under vigorous stirring for 10 min. The resultant solution containing NSs was incubated at room
temperature overnight.
Gold NSs with an average diameter of 40, 60 and 80 nm were prepared via a several-step
procedure, including NS synthesis and subsequent etching. The latter step was used to achieve a
close-to-spherical shape of the NSs. Seed gold NSs with dimensions of approximately 16 nm
were prepared using a method reported by the Tian and Gang groups6,,7. A solution of freshly
prepared, ice-cold NaBH4 in deionized water (10 mM, 0.6 mL) was added under stirring to a
solution prepared by mixing aqueous CTAB (0.1 M, 9.833 mL) and HAuCl4 (15 mM, 0.167 mL)
in a 20 mL scintillation vial, thereby forming a seed solution. After 2 min stirring, the pale
yellow seed solution was left undisturbed at room temperature for 2 h. This solution was then
diluted to 100 mL with deionized water. In another flask, growth solution was prepared by
combining aqueous solutions of CTAB (0.24 M, 4 mL) with HAuCl4 (15 mM, 0.133 mL) and
ascorbic acid (0.1 M, 3 mL). The resulting clear solution was subsequently diluted to 50 mL with
deionized water. To this solution, 0.6 mL of the diluted seed solution was added and the resulting
mixture was shaken vigorously, resulting in a pale pink-coloured solution. The resultant solution
was left undisturbed at room temperature for 12 h (to be used in the preparation of 40 nm-
diameter NSs) or 30 min (prior to the preparation of 60 nm- and 80 nm-diameter NSs). Excess
surfactant from this solution was removed by using one cycle of centrifugation (20000 g, 20 min,
28 °C), removing the supernatant and redispersing NS seeds with deionized water. Later in the
text, this solution is referred to as a "purified seed solution". To prepare 40, 60 and 80 nm-
26
diameter NSs, 5, 2.5, or 1 mL of the purified seed solution, respectively, were added to a 20 mL
scintillation vial and diluted it to 10 mL with deionized water. Then, 1.6 mL of aqueous
surfactant solution (0.1 M CTAB for 40 nm-diameter NSs, 0.1 M cetylpyridinium chloride
(CPC) for 60 nm and larger diameter NSs) was added to this solution. The resultant mixed
solution was heated to 30 ºC in a water bath for 5 min. The solution was removed from the water
bath and 250 μL of 15 mM aqueous HAuCl4 solution and 0.8 mL of 0.1 M ascorbic acid were
added in succession. The NSs were left undisturbed at room temperature for 15 min. This
synthesis was scaled up to obtain 100 mL of NS solution.
Gold NSs with a diameter in the range from 60 to 80 nm synthesized as described above,
did not have a spherical shape. To transform them into NSs, we used an etching procedure
reported by Zou et al8. Briefly, 10 mL of the as-synthesized NSs with a diameter of 60 or 80 nm
NSs were purified from excess surfactant using centrifugation (5000 g, 15 min, 28 °C), removal
of the supernatant and dilution with deionized water. Equal volumes of the purified solution of
gold NSs and 0.1M FeCl3 were combined and left for 2 h at room temperature with intermittent
stirring. The solution of NSs was then purified from excess reagents using two centrifugation
cycles (5000 g, 15 min, 28 °C), removal of the supernatant and dilution with an aqueous 50 mM
CPC solution. The NSs were then subjected to two rounds of centrifugation (5000 g, 15 min, 28
°C), removal of the supernatant and addition of deionized water to achieve the original NS
concentration. The resulting NSs exhibited a close-to-spherical shape.
2.2.1.2 Synthesis of gold spherocylindrical nanorods
Synthesis of spherocylindrical gold nanorods. Gold nanorods (NRs) were synthesized
using the procedure reported by the Murray group9. The seed solution was prepared by reducing
HAuCl4 (15 mM, 0.12 mL) in 2.5 mL of 0.2 M aqueous solution of CTAB with 0.6 mL of 10
27
mM cold NaBH4. The seeds were aged for 30 min. To prepare the growth solution, a 10 mL
aqueous solution of CTAB (8 mM) and sodium oleate (16 mM) was prepared in a 25 mL
Erlenmeyer flask, followed by addition of 0.29 mL of 10 mM AgNO3 solution. After 15 min
incubation at 30 °C, 10 mL of 1 mM HAuCl4 solution was added, followed by stirring the
mixture for 90 min at room temperature. At this point, 0.12 mL of concentrated HCl (12 M) was
added to the growth solution, followed by addition of 1.25 mL of 0.064 M ascorbic acid. To
initiate NR growth, 0.03 mL of the seed solution was injected into the growth solution, which
was stirred for 30 s and left undisturbed at 30 °C for 12 h. The resultant CTAB-stabilized NRs
had an average diameter and length of 16 ± 1 nm × 80 ± 7 nm, respectively.
2.2.1.3 Synthesis of gold nanodumbbells
The preparation of gold nanoparticles with a dumbbell shape (NDs) was achieved by
using a procedure described by Grzelczak et al10. CTAB (100 mM, 25 mL) was mixed with
HAuCl4 (50 mM, 0.125 mL) and stored for 5 min at 27 °C. Subsequently, KI (10 mM, 14.25 µL)
and ascorbic acid (100 mM, 0.1 mL) were added. Finally, 0.6 mL of a solution of as-synthesized
CTAB-functionalized sphero-cylindrical NRs (synthesized as described above) was used as a
seed solution and was added to the mixture under stirring. After 15 min, the synthesis of NDs
was complete. The resultant CTAB-stabilized NDs had a length of 92 ± 7 nm and the maximum
diameter (measured at the ND tip) of 34 ± 2 nm and the minimum diameter (measured at the ND
center) of 19 ± 2 nm.
2.2.1.4 Synthesis of silver nanocubes
Core-shell silver nanocubes with a gold core (referred to as silver NCs) were stabilized
with CPC and prepared using a three-step protocol reported elsewhere11. First, 3 nm-diameter
gold NSs were prepared by rapidly injecting 0.60 mL of ice-cold, freshly prepared 10 mM
28
NaBH4 solution into a rapidly stirred mixed aqueous solution of HAuCl4 (10 mM, 0.25 mL) and
CTAB (0.1 M, 9.75 mL). After stirring for 2 min, the solution was left undisturbed for 2 h and
then diluted to 100 mL with deionized water. Then, 0.6 mL of this solution was added under
stirring to a mixture of HAuCl4 (0.2 mL, 10 mM), CTAB (4 mL, 0.2 M), ascorbic acid (3 mL,
0.1 M) and 43 mL of deionized water. The reaction mixture was left undisturbed at room
temperature for 12 h, yielding a purple solution of octahedral gold nanoparticles (seeds)
approximately 16 nm in diameter. Two washing cycles using centrifugation at 15000 g for 15
min and separation of the supernatant were used to replace the surfactant-rich solution with
deionized water. In the final step, 2.5 mL of the solution of gold seeds, 7.4 mL of deionized
water and 1.6 mL of the aqueous 0.1 M solution of CPC were mixed in a 20 mL vial placed in an
oil bath at 60°C. This step was followed by the sequential addition of the aqueous solution of
AgNO3 (0.2 mL, 10 mM) and ascorbic acid (0.8 mL, 0.1 M) under stirring. After 1 h, the
reaction mixture was cooled in an ice-bath. The resulting NCs with the average length of the side
of 40 nm were washed twice via centrifugation at 8000 g for 10 min, separation of the
supernatant and redispersion in deionized water.
2.2.1.5 Synthesis of gold nanocubes
Gold NCs were synthesized using a modified seed-mediated method reported by Chen et
al12. A solution of freshly prepared, ice-cold NaBH4 in deionized water (10 mM, 0.6 mL) was
added under stirring to a solution prepared by mixing aqueous CTAB solutions (0.1 M, 9.835
mL) and HAuCl4 (15 mM, 0.167 mL) in a 120 mL Erlenmeyer flask. After 2 min stirring, the
pale yellow mixture was left undisturbed for 2 h at room temperature. This seed solution was
then diluted to 100 mL with deionized water. In another flask, a growth solution was prepared by
combining aqueous solutions of CTAB (0.24 M, 4 mL) with HAuCl4 (15 mM, 0.133 mL) and
ascorbic acid (0.1 M, 3 mL). The resulting clear solution was subsequently diluted to 50 mL with
29
deionized water. To this solution, 0.6 mL of the diluted seed solution was added. The resulting
mixture was vigorously shaken and then left undisturbed for 12 h at room temperature. Excess
CTAB from this solution was removed by one centrifugation cycle (15500 g, 20 min, 27 °C),
removing the supernatant and redispersing the nanoparticle precipitate in 50 mL of deionized
water.
To prepare 50 and 60 nm-diameter NCs, 30 and 20 mL of the purified seed solution,
respectively, was added to a 120 mL Erlenmeyer flask and diluted to 100 mL with deionized
water. Then, 16 mL of 0.1 M CTAB was added to this solution. The resultant mixed solution was
heated to 30 ºC in a water bath for 5 min. Following this step, the solution was removed from the
water bath and 2.5 mL of 15 mM aqueous HAuCl4 solution and 8 mL of 0.1 M ascorbic acid
were added in succession, under stirring for 5 min. The resulting NCs were left undisturbed at
room temperature overnight. Functionalization of nanoparticles with polymer ligands
2.2.1.6 Functionalization of nanoparticles with thiol-terminated polystyrene ligands
The solutions of as-synthesized NPs were concentrated from 1.5 mL solution to
approximately 30 μL using 15 min centrifugation at 27 °C (see Table 2 for centrifugation speed)
and subsequent removal of the supernatant. The concentrated NP solution was sonicated for 5 s
and added to 1.5 mL of the solution of thiol-terminated polystyrene in tetrahydrofuran (THF)
(see Table 2). The resulting solution was maintained undisturbed at room temperature overnight.
Then, the NPs were separated from free (non-attached) PS via ten cycles of centrifugation (six in
the case of silver NCs) of the solution (see Table 2, 15 min, 20 °C), removal of the supernatant,
and dilution of the solution with THF. Extinction spectra of the PS-stabilized NSs were acquired
using a Cary 5000 UV-VIS-NIR spectrometer.
30
Table 2. Solutions of thiol-terminated polystyrene used for ligand exchange
* Molecular weight of thiol-terminated polystyrene with dispersity in the range of 1.05-1.09 was
provided by Polymer Source Inc.
2.2.1.7 Determination and variation of polymer grafting density
To examine the effect of grafting density on polymer surface segregation, polymer
adsorption to the NS surface was adjusted by changing the ratio of polymer molar concentration
to the surface area of the NSs, CPS/SA, in the ligand exchange procedure. The value of CPS/SA, in
units of mol/nm2, was determined as
Type of metal NPs
Molecular weight,
Mn, of thiol-
terminated PS
(g/mol)*
Concentration of
thiol-terminated PS in
THF (mg/mL)
Centrifugation speed
(g)
20 nm gold NSs 30 000 0.12 20000
20 nm gold NSs 50 000 0.20 20000
40 nm gold NSs 30 000 0.24 7000
40 nm gold NSs 50 000 0.47 7000
60 nm gold NSs 50 000 0.50 6000
80 nm gold NSs 50 000 0.60 5000
Gold NCs 50 000 0.50 6000
Silver NCs 50 000 0.50 5000
Gold NDs 50 000 2.0 7000
Gold NRs 50 000 0.55 6000
31
𝐶PS/SA = 𝐶PS
𝜋𝐷2𝐶NS𝑁Av (1),
where CPS is the concentration of PS-50K (mol L-1) in the solution, CNS is the concentration of
gold NSs in the solution, D is the average diameter of gold NS (determined by analyzing TEM
images) and NAv is the Avogadro number (NAv= 6.022 x 1023 mol-1). The concentration of NSs
was determined by measuring their extinction in the solution in THF using as CNS = A/εl, where
A is the absorbance value, ε is the molar absorptivity of the NSs (Lmol-1cm-1), and l is the path
length (cm) (Table 3).
Table 3. Molar absorptivity of gold NSs with different diameters
NS diameter, D (nm) Molar absorptivity, ε (x 107 L mol-1 cm-1) Reference
20 1.1 5
40 9.9 13
60 35 13
80 77 13
The grafting density of PS-50K on the surface of gold NSs was determined by
thermogravimetric analysis (TGA). Temperature-weight loss curves were obtained under
nitrogen atmosphere using an SDT Q600 TGA from TA Instruments. The temperature was
ramped at a heating rate of 10 °C/ min from 50 to 800 °C. The onset temperature of PS
degradation was 380 °C, while the temperature of 50% mass loss of the polymer was 420 °C,
32
consistent with previously reported values14. Representative TGA traces are shown in Figure
2.2.1.
Figure 2.2.1. Representative thermal gravimetric analysis traces. Thermal gravimetric
analysis traces for 20 nm-diameter gold NSs stabilized with PS-50K. The ligand exchange
procedure was conducted at the ratio of PS-50K concentration-to-NS surface area of 2.6 x10-23
(bottom trace), 2.6 x 10-24 (middle trace) and 2.6 x 10-25 mol/nm2 (top trace).
Approximately 4-6 mg of PS-50K-capped gold NSs were analyzed in each experiment.
The average grafting densities determined from two to four experiments for the different
values of CPS/SA are summarized in Table 3. The grafting density of PS-50K was calculated using
the method reported by Benoit et al15 as
=
4
3(
𝑤𝑡shell𝑤𝑡core
)𝜌core(𝐷
2)
3 𝑁Av
MW𝜋𝐷2 (2),
33
where wtshell and wtcore are the weight fractions (in %) of the polymer layer and the gold NSs,
respectively, in the NS sample, determined by TGA, ρcore is the density of gold of 19.3 g cm-3 16,
and MW is the molecular weight of PS ligands (Mn=50,000 g/mol).
Table 4. Variation in polymer grafting density with polymer concentration in the solution
in ligand exchange process.
NS diameter, D
(nm) Polymer concentration (mol/nm2) Grafting density, σ (chains/nm2)
20
3.0E-23 0.083 ± 0.0340
3.0E-24 0.085 ± 0.0400
3.0E-25 0.031 ± 0.0260
40
1.9E-23 0.042 ± 0.0019
1.9E-24 0.031 ± 0.0015
1.9E-25 0.012 ± 0.0086
4.2E-26 0.003 ± 0.0010
60
2.4E-23 0.022 ± 0.0006
2.4E-24 0.017 ± 0.0063
2.4E-25 0.011 ± 0.0026
2.4E-26 0.003 ± 0.0005
80
2.5E-23 0.029 ± 0.0028
2.5E-24 0.021 ± 0.0057
2.5E-25 0.012 ± 0.0058
34
2.5E-26 0.004 ± 0.0021
* The error bars represent the average of two to four TGA measurements. The PS-50K grafting
time was 12 h.
2.2.1.8 Functionalization of NSs with thiol-terminated poly(4-vinyl pyridine) ligands
The solution of as-synthesized 20 nm-diameter gold NSs was concentrated from 1.5 mL to
approximately 30 μL using centrifugation (15000 g, 30 min, 27 °C) and subsequent removal of
the supernatant. The concentrated NS solution was sonicated for 5 s and diluted with 15 mL of
water. 1 mL of the NS solution was then dispersed in 10 mL of a solution of thiol-terminated
poly(4-vinyl pyridine in DMF (2.5 mg/mL). The resulting solution was maintained undisturbed
at room temperature overnight. Then, the NSs were separated from free poly(4-vinyl pyridine via
five cycles of centrifugation of the solution (12000 g, 15 min, 27 °C), removal of the
supernatant, and dilution of the solution with DMF. The resulting solution of gold NSs in DMF
(11 mL) was concentrated to 1 mL by centrifugation and dispersed in 10 mL of 0.01M HCl
solution. The solution was purified by 2 cycles of centrifugation (12000 g, 15 min, 27 °C),
followed by NS redispersion in 0.01M HCl solution.
2.2.1.9 Functionalization of NSs with poly(N-vinyl carbazole)
The solution of as-synthesized gold NSs was concentrated from 1.5 to ~30 μL using 15
min centrifugation at 27 °C (see Table 2 for centrifugation speed), and subsequent removal of the
supernatant. The concentrated NS solution was sonicated for 5 s and added to 1.5 mL of the 20
mg/mL solution of thiol-terminated poly(N-vinyl carbazole) dissolved in THF. The resulting
solution was sonicated for 10 min and subsequently left undisturbed at room temperature
overnight. Then, the NSs were separated from free poly(N-vinyl carbazole via six cycles of 15
35
min-long centrifugation cycles at 20 °C and 20000 g, removal of the supernatant, and dilution of
the solution with THF.
2.2.1.10 Functionalization of NSs with PS-co-PI copolymer
To remove excess CTAB, a solution of as synthesized 32-nm-diameter NSs water was
centrifuged (7000 g, 15 min, 27 °C), the supernatant was removed and the sediment was
redispersed in deionized water. Subsequently, 10 mL of the NS solution was centrifuged (7000 g,
10 min, 27 °C) and redispersed in 10 mL of 1 mg/mL solution of PS-co-PI in THF. After
incubating the resulting solution for 18 h, the excess of unreacted PS-co-PI was removed by ten
cycles of centrifugation (7000 g, 10 min, 27 °C), each followed by the removal of supernatant
and redispersion in THF.
2.2.2 Polymer surface segregation experiments
2.2.2.1 Sample preparation for imaging
To prepare samples of polymer-segregated NPs for TEM imaging, a droplet of the NP
solution was deposited onto a 300 mesh carbon-coated copper grid and left for 90 s, unless
otherwise specified. The remaining solution was removed with a Kimwipe tissue. TEM images
were obtained using a Hitachi S-7000 microscope at 75-100 kV with a filled liquid nitrogen cold
trap. Images were taken in different areas of the grid. Each experiment was repeated in triplicate
with over 100 individual NP species imaged.
2.2.2.2 Polystyrene segregation on the surface of gold NSs.
Polymer-capped NSs dispersed in THF were dried in a vial using a stream of air. Once the
THF evaporated, the NSs were redispersed with 500 μL of DMF and sonicated for 5 s to prepare
a 0.3 nM solution of gold NSs in DMF. Then, 500 μL of a DMF/water mixture at Cw=8 vol. %
was added dropwise to the NS solution in DMF under gentle swirling of the vial, to reach Cw=4
36
vol. %. The vial was sealed and maintained in a water bath at 40 °C for 1-24 h, with no
significant change in polymer surface structure for this time interval. Unless specified, the
experiments were carried out for 24 h.
2.2.3 UV crosslinking of segregated polymer on the surface of gold NSs
To induce polymer surface segregation on gold 35 nm-diameter NSs, a DMF/water
solution (200 μL, Cw=2 vol. %) was added to 200 μL of the solution of PS-co-PI-functionalized
gold NSs in DMF to obtain the final concentration of water of Cw=1 vol. %. The solution was
sonicated for 5 sec and then immersed in a bath at 40 °C for 18 h to generate surface patches.
Subsequently, 100 μL of the patchy NSs was added to 100 μL of a solution of
azobisisobutyronitrile (AIBN) solution (0.1 wt. %) in the DMF/water mixture at Cw=1 vol. %.
The suspension was incubated at 4 °C for 4 h to allow AIBN to diffuse into the copolymer
patches. Subsequently, the NS solution was exposed to ultra-violet illumination (UV-A lamp,
Honle, UVAPrint 40C, λ=365 nm, I=30 mW cm−2) for 5 min. Under these conditions, the AIBN
radicals initiated crosslinking of the 3,4-polyisoprene units of the PI block. To demonstrate
permanent crosslinking of the PS-co-PI patches, 100 μL of the resulting NS solution was diluted
in 1 mL of THF, a good solvent for the PS-co-PI copolymer, sonicated for 15 min and incubated
at room temperature for 24 h.
2.2.4 Measurement of wetting angle of polystyrene solution on gold surfaces
A 40 μM solution of polystyrene in DMF was prepared. To 500 μL of this solution, 500
μL of 8 vol. % water in DMF was added dropwise to reach the total concentration of water CW
=4 vol. %. After ca. 15 min of stirring, the stir bar was removed and the solution was incubated
at room temperature for 48 h. The solution separated into two phases, namely, a solvent-rich
37
phase (the top phase) and a PS-rich phase (the bottom phase). Using a micropipette, 5 μL of the
PS-rich phase was deposited onto a silica wafer coated with a 50 nm-thick gold layer. The
wetting angle of the PS-DMF-water solution on the gold surface was measured using a DSA-100
Krüss drop-shape analyzer. The average static wetting angle, determined from five independent
measurements, was 15.1±0.7º and 12.1±0.4º, for the solution of PS-30K and PS-50K,
respectively, which corresponded to a surface tension of 194±49 mN/m for PS-30K and 302± 29
mN/m for PS-50K as determined by drop-shape analysis using a Young-Laplace fit17.
38
2.3 References
1. S. K. Varshney, X.-F. Zhong, A. Eisenberg, Anionic homopolymerization and block of 4-
vinylpyridine and its investigation by high-temperature size-exclusion chromatography in N-
methyl-2-pyrrolidinone. Macromolecules 26, 701 (1993). DOI: 10.1021/ma00056a022
2. C.-F.Huang, J. A.Yoon, K. Matyjaszewski, Synthesis of N-vinylcarbazole-N-vinylpyrrolidone
amphiphilic block copolymers by xanthate-mediated controlled radical polymerization. Can. J.
Chem., 88, 228 (2010). DOI: 10.1139/v09-160
3. G. Moad, E. Rizzardo, S. H. Thang, Living radical polymerization by the RAFT process. Aust.
J. Chem. 58, 379 (2005). DOI: 10.1071/CH05072
4. D. L. Patton, M. Mullings, T. Fulghum, R. C. Advincula, A facile synthesis route to thiol-
functionalized α, ω-telechelic polymers via reversible addition fragmentation chain transfer
polymerization. Macromolecules 38, 8597 (2005). DOI: 10.1021/ma051035s
5. N. R. Jana, L. Gearheart, C. J. Murphy, Seeding growth for size control of 5−40 nm diameter
gold nanoparticles. Langmuir 17, 6782 (2001). DOI: 10.1021/la0104323
6. F.-R. Fan et al., Epitaxial growth of heterogeneous metal nanocrystals: from gold nano-
octahedra to palladium and silver nanocubes. J. Am. Chem. Soc. 130, 6949 (2008). DOI:
10.1021/ja801566d
7. F. Lu et al., Discrete nanocubes as plasmonic reporters of molecular chirality. Nano Lett. 13,
3145 (2013). DOI: 10.1021/nl401107g
8. R. Zou et al., Selective etching of gold nanorods by ferric chloride at room temperature.
CrystEngComm. 11, 2797 (2009). DOI: 10.1039/B911902G
9. X. Ye, C. Zheng, J. Chen, Y. Gao, C. B. Murray, Using binary surfactant mixtures to
simultaneously improve the dimensional tunability and monodispersity in the seeded growth of
gold nanorods. Nano Lett. 13, 765 (2013). DOI: 10.1021/nl304478h
39
10. M. Grzelczak et al., Steric hindrance induces crosslike self-assembly of gold nanodumbbells.
Nano Lett. 12, 4380 (2012). DOI: 10.1021/nl3021957
11. A. Klinkova et al., Structural and optical properties of self-assembled chains of plasmonic
nanocubes. Nano Lett. 14, 6314 (2014). DOI: 10.1021/nl502746h
12. H. Chen et al., Plasmon coupling in clusters composed of two-dimensionally ordered gold
nanocubes. Small 5, 2111 (2009). DOI: 10.1002/smll.200900256
13. Gold Nanoparticles: Properties and Applications (Sigma-Aldrich technical report;
http://www.sigmaaldrich.com/materials-science/nanomaterials/gold-nanoparticles.html).
14. M. C. Costache, C. A. Wilkie, High-throughput method for estimating the time to sustained
ignition of polystyrene-clay nanocomposites based on thermogravimetric analysis. Polym. Adv.
Technol. 21, 506 (2010). DOI: 10.1002/pat.1460
15. N. Benoit et al., Measuring the grafting density of nanoparticles in solution by analytical
ultracentrifugation and total organic carbon analysis. Anal. Chem. 84, 9238–9245 (2012). DOI:
10.1021/ac301980a
16. W. M. Haynes, Ed., CRC Handbook of Chemistry and Physics. (CRC Press, Boca Raton, FL,
ed. 95, 2014).
17. H.-J. Butt, K. Graf, M. Kappl, Physics and Chemistry of Interfaces. (Wiley, 2003).
40
Chapter 3
Structural Transitions in Nanoparticle Assemblies Governed by
Competing Nanoscale Forces
Partially reprinted with permission from: Choueiri, R. M., Klinkova, A., Thérien-Aubin, H.,
Rubinstein, M. & Kumacheva, E. Structural Transitions in Nanoparticle Assemblies Governed
by Competing Nanoscale Forces. J. Am. Chem. Soc. 135, 10262–10265 (2013). DOI:
10.1021/ja404341r
Contribution: R. M. Choueiri designed and carried out all experiments and data analysis and
contributed to the interpretation and preparation of the manuscript.
3.1 Introduction
Ensembles of inorganic nanoparticles (NPs) show collective electronic, optical and
magnetic characteristics that originate from the coupling of size- and shape-dependent properties
of individual NPs1,2. A cost-efficient approach to the fabrication of nanoscale materials utilizes
the self-assembly of NP building blocks and yields a broad range of nanostructures from small
clusters to large three-dimensional (3D) lattices3,4. Control over the size and shape of these
assemblies relies on our understanding of the nanoscale forces acting between NPs5. Examples of
such forces include dipole−dipole interactions6, electrostatic forces7, hydrogen bonding8, hydro-
phobic9, coordination10 and biospecific forces11. Attractive and repulsive interactions between
NPs may change in magnitude and compete, depending on environmental factors. Generation of
different types of nanostructures from the same type of NPs offers a route to the fabrication of
stimuli- responsive nanomaterials. Typically, this strategy relies on regiospecific attachment of
41
distinct ligands to NP surface. For example, gold and semiconductor nanorods capped with
different molecules along their long side and at the tips behave as amphiphilic molecules and
organize in a variety of structures, when the quality of the solvent is selectively reduced for a
particular ligand type4,12. Site-specific attachment of distinct ligands to the NP surface is
achieved due to their preferential binding to a particular crystalline facet13, which may be
challenging for small, close-to-spherical NPs and/or the use of macromolecular ligands. A more
beneficial approach to distinct self-assembled nanostructures would rely on stimulus-dependent
competition of nanoscale forces including electrostatic repulsion, hydrophobic attraction,
magnetic and biospecific interactions. Here we report this approach by inducing structural
transitions between two distinct types of nanostructures: linear and 3D assemblies of charged,
polymer-stabilized gold NPs. The transitions were governed by the competition of attractive
hydrophobic/poor solvency forces and repulsive electrostatic forces, and were achieved by
varying the composition and ionic strength of the solvent or the molecular weight of the polymer
ligands.
3.2 Results and Discussion
3.2.1 Self-assembly experiments
Spherical 23 nm-diameter gold NPs stabilized with cetyltrimethylammonium bromide
were synthesized and subjected to ligand exchange with thiol-terminated polystyrene (PS)
molecules14, as described in section 2.2.1.1 and section 2.2.1.6. The number average molecular
weights of the PS ligands used in the present work were 5 × 103, 1.2 × 104, 2 × 104, 3 × 104 and 5
× 104 g/mol (later in the text referred to as PS-5K, PS-12K, PS-20K, PS-30K and PS-50K,
respectively). Following the ligand exchange procedure conducted in tetrahydrofuran (THF), the
NPs were redispersed in N, N-dimethylformamide (DMF). A solution of NPs in DMF was
42
colloidally stable for at least 5 months. The self-assembly of the NPs was induced by adding
various amounts of water to the solution of NPs in DMF, thereby reducing the quality of solvent
for the PS ligands15. To achieve this, the solvent quality was reduced by adding the water in a
mixture with DMF dropwise to the solution of PS-functionalized NPs in DMF while swirling the
reaction container. The slow addition was employed to prevent the formation of kinetically
trapped structures. To screen unfavorable interactions between the solvent and the PS molecules,
the NPs associated in various types of structures. For the self-assembly time tSA = 1h, we
observed three types of species in the colloidal solution depending on the content of water, Cw, in
the DMF/water mixture and the molecular weight, Mn, of the PS ligands: individual NPs,
spherical 3D NP clusters (globules), and linear NP chains with occasional branches (Figure
3.2.1).
43
Figure 3.2.1 Transmission electron microscopy (TEM) images of the representative NP
species observed in NP solution in DMF-water mixture. (a) Individual NPs stabilized with
PS-5K at Cw = 5 vol. %. (b) Globules of NPs stabilized with PS-50K at Cw = 5 vol. %. (c)
Mixture of chains and globules of NPs stabilized with PS-50K at Cw = 10 vol. %. (d) Chains of
NPs stabilized with PS-5K at Cw = 15 vol. %. The scale bars are 100 nm. The self-assembly time
tSA is 1 h.
For a particular range of Cw and Mn, coexistence of chains and globules of NPs was
observed. The self-assembly of the NPs was “mapped” by a phase-like diagram in the Mn− Cw
parameter space. The diagram in Figure 3.2.2 shows the transitions between individual NPs (I),
44
globules (G) and NP chains (C), as well as a narrow G + C range. No self-assembly occurred for
the NPs stabilized with low-molecular weight PS and/or in the low Cw range due to insufficient
enthalpy gain to compensate for the loss in entropy. The ability of NPs to form globules vs.
chains was determined by the interplay of Mn and Cw. At high Mn (≥30000) and low Cw the
formation of globules was favored, whereas for higher content of water (dependent on Mn), NP
chains were the dominant species. The transition G → C was not sharp and occurred via a regime
(C+G), in which both of the species were present.
Figure 3.2.2 Phase-like diagram of the self-assembled structures of gold NPs, plotted in the
Mn− Cw space with tSA= 1 h. . The diagram was constructed by analyzing TEM images.
45
The dissociation of self- assembled structures (globules or chains) to individual NPs was
achieved by adding DMF to achieve Cw = 1.6 vol. % with either sonication for 15 min or heating
of the system at 40 °C for 4 h (Figure 3.2.3).
Figure 3.2.3 Testing the reversibility of self-assembled chains and globules of gold
nanoparticles. (a) Testing reversibility of the formation of NP globules at Cw = 5 vol. % by
adding DMF to the colloidal solution of globules to achieve Cw = 1.7 vol. %. (b) Testing
structural transitions of NP chains at Cw = 15 vol. % by adding a DMF to the colloidal solution
of chains to achieve Cw = 1.7 vol. %. tSA=1 h. The NPs were stabilized with PS-50K.
We carried out a detailed study of the NP chains and globules. The globular assemblies
were similar to 3D clusters that were formed in tetrahydrofuran by PS-stabilized gold NPs16.
46
Similar to this earlier report, in our work, the self-assembly of NP globules in solution could be
monitored by dynamic light scattering and could be quenched by encapsulating the globules with
a polystyrene-block-poly(acrylic acid) copolymer. The hydrodynamic diameter of the globules
measured after self-assembly time, tSA = 1 h in the DMF/water mixture (Cw = 7.5 vol. %) was
103 ± 2 nm, in comparison with 32.8 ± 3.4 nm, measured for individual NPs in DMF solution
(Figure 3.2.4a). The corresponding change in the hydrodynamic radius of the globules
(determined in dynamic light scattering experiments) is shown in Figure 3.2.4b. With increasing
concentration of water in the DMF/water mixture, the diameter of the globules (determined by
analyzing TEM images) increased from ∼72 nm (Cw = 5 vol. %) to ∼84 nm (Cw = 7.5 vol. %).
Chains of NPs grew in a marked similarity to step-growth polymerization, with the
average number of NPs in the chains increasing linearly with time, similar to our earlier work on
the self-assembly of gold nanorods17,18. The average distance, d, between the NPs in the chains
increased with the molecular weight of PS ligands (Figure 3.2.4c), consistent with a larger
polymer volume localized in the gap between the NPs. The same trend was observed for the
dense NP arrays obtained by drying their solution in DMF on the TEM grid. For such arrays, the
interparticle distance was consistently larger than that in the chains (Figure 3.2.4c), due to the
more extended vitrified conformation of PS ligands in a good solvent. For 10 ≤ Cw ≤ 30 vol. %,
interparticle distance did not significantly change (Figure 3.2.4d), in agreement with our earlier
work14.
47
Figure 3.2.4 Characterization of globules and the interparticle spacing of nanoparticles
functionalized with PS. (a) Variation in the hydrodynamic diameter of the globules formed by
NPs capped with PS-50K at Cw =7.5 vol. %, measured at tSA of 0, 20, 30, 40, 50, and 60 min,
shown with blue, red, yellow, green, violet, and light blue curves, respectively. (b)
Hydrodynamic diameters of globules obtained from NPs stabilized with PS-50K at Cw = 5 vol. %
(red curve) and at Cw = 7.5 vol. % (blue curve). (c) Variation in interparticle distance in the
chains at Cw = 15 vol. % (red diamonds) and in the close-packed arrays of NPs dried on the TEM
grid at Cw =0 (blue squares), plotted as a function of Mn of PS ligands. (d) Variation in
interparticle spacing plotted vs. Cw in the DMF/water mixture for the chains formed by NPs
stabilized with PS-5K. The dashed red line shows the corresponding interparticle distance in
close-packed arrays dried on the TEM grid at Cw =0, good solvent conditions.
The structural transitions shown in Figure 3.2.2 were reversible. For example, for NPs
stabilized with PS-50k, the C → I and G → I transformations occurred when the Cw was reduced
48
to ∼2 vol. %. Importantly, the C→I transition occurred via the globular state, as shown in the
phase diagram.
3.2.2 Competition of nanoscale forces
The structural transitions between NP chains and globules were rationalized as follows.
The resultant structure of NP assemblies is determined by the minimization of the total energy of
the system, ΔEt, governed by the contributions of attraction forces (favoring a compact globular
structure) and repulsion forces (favoring the formation of chains) as
ΔEt = ΔEel + ΔEh (3)
In eq 1, ΔEel is the change in energy of repulsive electrostatic interactions and ΔEh is the change
in energy representing a combined effect of hydrophobic/poor solvency attraction forces (later in
the text referred to as “hydrophobic forces”). We ignored attractive short-range van der Waals
interactions, since over the entire range of interparticle separations hydrophobic forces
dominated. A weak contribution of van der Waals forces was also supported by the absence of
association of PS-stabilized NPs at low content of water in the DMF/water mixture. Figure
3.2.5a and b illustrates the configurations formed by three adjacent NPs in the chains and
globules, respectively. The type of structure was determined by the angle θ between the lines
connecting the center of the second NP with the centers of adjacent 1st and 3rd NPs. The value
of θ changed from 60° (the globular shape) to 180° (the chain shape)
The change in energy of attraction upon NP association was calculated as
ΔEh = ΔEst = ΔA (4)
where ΔEst is the reduction in the surface energy when two NPs form contact, γ is the interfacial
tension between the collapsed PS ligands and the DMF/water mixture, and ΔA is the change in
49
area of this interface when two NPs associate. The change in electrostatic energy for two
associating NPs in the absence of added salt was calculated using the approximation developed
for charged spherical NPs functionalized with polymer brushes19 as a logarithmic function of the
surface-to-surface interparticle distance as
RL
RDR
lE
2
22ln
kT
2 B
2
el
(5)
where R is the radius of the gold core of the NP; D is the center-to-center distance between the
two neighboring PS- coated NPs; L is the center-to-center distance between the 1st and the 3rd
PS-coated NPs (equal to 2D and D for θ of 180 and 60°, respectively), as shown in Figure 3.2.5a,
and b; k is the Boltzmann constant and T is the absolute temperature. In Equation 5, lB is the
Bjerrum length (=e2/εkT), the distance at which the total interaction potential of counterions is
equal to the thermal energy kT, where e is the elemental charge and ε is the dielectric constant of
the solvent. The NP configuration at θ = 180° was chosen as a reference. Thus Equation 5 shows
a logarithmic dependence of the electrostatic energy as a function of the surface-to-surface
separation between NPs with respect to the 180° configuration, with separation between surfaces
2D-2R.
We evaluated the variation in ΔEt for the assembly of NPs stabilized with PS-50K in two
solvent DMF/water mixtures at Cw of 5 and 15 vol. %, in which the NPs assembled into the
globules and the chains, respectively. For calculations, we used R = 11.5 nm, and ε of 40 and 49
20 (corresponding to lB of 1.14 and 1.4 nm, respectively), and γ of 1.4 × 10−3 J/m2 to 1.9 × 10−3
J/m2 for Cw of 5 and 15 vol. %, respectively. The derivation of Equation 5 and details of the
calculations are given in Appendix 1.
50
Figure 3.2.5c shows the variation in ΔEt for two solvent compositions. For Cw = 5 vol. %,
ΔEt ≈ - 20 kT at θ =60° while ΔEt ≈ 0 kT at θ =180°. Thus, the NP configuration with a
minimum in ΔEt at θ =60° was favored (corresponding to NP globules), due to the stronger
contribution of the surface energy. For Cw = 15 vol. %, the deeper minimum in ΔEt was achieved
at θ = 180° corresponding to NP chains with ΔEt ≈ 0 kT in comparison to ΔEt ≈ 14 kT for NP
globules, due to the larger contribution of electrostatic repulsion. Both predictions were in
agreement with experimental results. We note that individually, both types of nanostructures
have been reported for gold NPs, although in different systems. Chains were observed for
charged gold NPs stabilized with low-molecular- weight ligands21,22, while the “non-directional”
attraction between PS- or DNA-capped gold NPs led to the formation of 3D lattices3. The
schematic in Figure 3.2.5d summarizes the structural transitions in assemblies of gold NPs. Poor
solvency conditions in conjunction with strong electrostatic repulsion yielded NP chains. Poor
solvency conditions and weak electrostatic forces yielded globules. Improvement in the quality
of solvent and reduction in Eel led to the C → G transition. When the quality of solvent was not
sufficiently reduced, the NPs existed as individual species.
51
Figure 3.2.5 Schematics of nanoparticle assembly and angle-dependence of nanoscale
forces. (a,b) Schematics of three neighboring NPs in the chain (a) and in the globule (b). The
inner and outer circles illustrate the metal Au cores and the PS shells, respectively. For notations
see Appendix 1. (c) Change in the total energy of the system with angle θ between the lines
connecting the center of the second NP with the centers of the 1st and 3rd NPs (shown in b). Red
and black lines correspond to the DMF/water mixtures with Cw of 5 and 15 vol. %, respectively.
The energy barrier between the globular and chain configurations is to be determined. (d)
Schematic of the structural transitions in NP ensembles.
Reduction in the quality of solvent did not lead to G → I transitions, because of the
kinetic trapping of globular structures. We note that in the self-assembly experiments, the
addition of water to the NP solution in DMF increased (although at a different rate) both Eh and
52
Eel, due to the reduction in solvent quality for the PS ligands and the increase in solvent polarity,
respectively.
Figure 3.2.6 Fraction of population of globules and chains formed by gold NPs stabilized
with PS-50K at Cw of 5, 10, and 15 vol. % with tSA=1h.
3.2.3 Validation of the proposed concept
To validate the argument about competing Eh and Eel, we conducted NP self-assembly in
the DMF/water mixture in the presence of NaCl. The addition of NaCl to the DMF/ water
mixture reduced the quality of solvent for the PS ligands23 and compressed the double electric
layer interactions of the NPs24, thus favoring “non-directional” hydrophobic forces and the
formation of globules. As expected, with increasing content of NaCl in the DMF/water mixture,
53
C → G transitions took place, with an intermediate regime, in which both species were present
(Table 5).
Table 5. Effect of the addition of NaCl and THF on nanoparticle assembly
CNaCl (mM) φTHF/DMF
10 50 300 0.1 0.5 1
Structure C C+G G C+G G G
In the second series of validation experiments, the competition between the hydrophobic
and electrostatic forces was further adjusted by adding tetrahydrofuran (THF) to the NP solution
in the DMF/water mixture at THF/ DMF volume ratios of 0.1 ≤ ϕTHF/DMF ≤ 1. Since the Flory−
Huggins interaction parameters for PS-DMF and PS-THF solutions are 0.46 and 0.4,
respectively21,25,26., in the presence of THF, the quality of the solvent for the PS ligands was
improved. Thus, the hydrophobic interactions between the PS-coated NPs in the
DMF/THF/water solution were weaker than in the DMF/water mixture. On the other hand, the
addition of THF reduced the polarity of the mixed solvent (the dielectric constant of THF is 7.5
27, where the dielectric constants of water and DMF are 79 and 37, respectively20,27). Thus,
electrostatic interactions between the NPs were suppressed. With increasing ratio ϕTHF/DMF in the
solution, the structure of NP assemblies changed from globules + chains (ϕTHF/DMF = 0.1, ε = 42)
to globules (ϕTHF/DMF =1, ε = 31) (Table 5).
54
3.3 Conclusions
To conclude, we have demonstrated reversible structural transitions in assemblies of
polymer-coated gold NPs. We show that by tuning the delicate balance between the hydrophobic
and electrostatic forces, assembly of NPs in two distinct configurations—3D globules or linear
chains—can be achieved. A particular nanostructure can be realized in several ways: by tuning
solvent polarity, by adding a salt, and by varying the molecular weight of the polymer ligands.
Since the formation of chain or globular assemblies was governed by the interaction of ligands,
rather than affected by the nature of the gold core, we hypothesize that the reported behavior is
general and similar transitions can be expected for other types of charged NPs coated with
polymer ligands.
55
3.4 References
1. (a) Nie, Z., Petukhova, A. & Kumacheva, E. Properties and emerging applications of self-
assembled structures made from inorganic nanoparticles. Nat. Nanotechnol. 5, 15–25 (2010).
DOI: 10.1038/nnano.2009.453 (b) Wang, L., Xu, L., Kuang, H., Xu, C. & Kotov, N. A. Dynamic
Nanoparticle Assemblies. Acc. Chem. Res. 45, 1916–1926 (2012). DOI: 10.1021/ar200305f (c)
Romo-Herrera, J. M., Alvarez-Puebla, R. A. & Liz-Marzán, L. M. Controlled assembly of
plasmonic colloidal nanoparticle clusters. Nanoscale 3, 1304 (2011). DOI: 10.1039/c0nr00804d
2. (a) Gao, Y. & Tang, Z. Design and Application of Inorganic Nanoparticle Superstructures:
Current Status and Future challenges. Small 7, 2133–2146 (2011). DOI:
10.1002/smll.201100474 (b) Carbone, C. et al. Self-Assembled Nanometer-Scale Magnetic
Networks on Surfaces: Fundamental Interactions and Functional Properties. Adv. Funct. Mater.
21, 1212–1228 (2011). DOI: 10.1002/adfm.201001325 (c) Xu, L. et al. Regiospecific Plasmonic
Assemblies for in Situ Raman Spectroscopy in Live Cells. J. Am. Chem. Soc. 134, 1699–1709
(2012). DOI: 10.1021/ja2088713
3. (a) Nykypanchuk, D., Maye, M. M., van der Lelie, D. & Gang, O. DNA-guided crystallization
of colloidal nanoparticles. Nature 451, 549–52 (2008). DOI: 10.1038/nature06560 (b) Park, S. Y.
et al. DNA-programmable nanoparticle crystallization. Nature 451, 553–6 (2008). DOI:
10.1038/nature06508
4. (a) Petukhova, A. et al. Standing arrays of gold nanorods end-tethered with polymer ligands.
Small 8, 731–7 (2012). DOI: 10.1002/smll.201101297 (b) Zhao, N., Liu, K., Greener, J., Nie, Z.
& Kumacheva, E. Close-Packed Superlattices of Side-by-Side Assembled Au-CdSe Nanorods.
Nano Lett. 9, 3077–3081 (2009). DOI: 10.1021/nl901567a (c) Zhao, N. et al. Self-Assembly of
Single-Tip Metal-Semiconductor Nanorods in Selective Solvents. Angew. Chemie Int. Ed. 50,
4606–4610 (2011). DOI: 10.1002/anie.201004915 (d) Fava, D., Nie, Z., Winnik, M. A. &
Kumacheva, E. Evolution of Self-Assembled Structures of Polymer-Terminated Gold Nanorods
in Selective Solvents. Adv. Mater. 20, 4318–4322 (2008). DOI: 10.1002/adma.200702786
56
5. Min, Y., Akbulut, M., Kristiansen, K., Golan, Y. & Israelachvili, J. The role of interparticle
and external forces in nanoparticle assembly. Nat. Mater. 7, 527–538 (2008). DOI:
10.1038/nmat2206
6. Thomas, J. R. Preparation and Magnetic Properties of Colloidal Cobalt Particles. J. Appl.
Phys. 37, 2914 (1966). DOI: 10.1063/1.1782154
7. Kalsin, A. M. et al. Electrostatic Self-Assembly of Binary Nanoparticle Crystals with a
Diamond-Like Lattice. Science. 312, 420–424 (2006). DOI: 10.1126/science.1125124
8. Mayer, C. R., Neveu, S., Secheresse, F. & Cabuil, V. Supramolecular assemblies of gold
nanoparticles induced by hydrogen bond interactions. J. Colloid Interface Sci. 273, 350–355
(2004). DOI: 10.1016/j.jcis.2004.01.063
9. Rasch, M. R. et al. Hydrophobic Gold Nanoparticle Self-Assembly with Phosphatidylcholine
Lipid: Membrane-Loaded and Janus Vesicles. Nano Lett. 10, 3733–3739 (2010). DOI:
10.1021/nl102387n
10. Si, S., Raula, M., Paira, T. K. & Mandal, T. K. Reversible Self-Assembly of Carboxylated
Peptide-Functionalized Gold Nanoparticles Driven by Metal-Ion Coordination. ChemPhysChem
9, 1578–1584 (2008). DOI: 10.1002/cphc.200800121
11. Caswell, K. K., Wilson, J. N., Bunz, U. H. F. & Murphy, C. J. Preferential End-to-End
Assembly of Gold Nanorods by Biotin−Streptavidin Connectors. J. Am. Chem. Soc. 125, 13914–
13915 (2003). DOI: 10.1021/ja037969i
12. Figuerola, A. et al. End-to-End Assembly of Shape-Controlled Nanocrystals via a
Nanowelding Approach Mediated by Gold Domains. Adv. Mater. 21, 550–554 (2009). DOI:
10.1002/adma.200801928
13. Murphy, C. J. et al. Anisotropic Metal Nanoparticles: Synthesis, Assembly, and Optical
Applications. J. Phys. Chem. B 109, 13857–13870 (2005). DOI: 10.1021/jp0516846
14. Nie, Z. et al. Self-assembly of metal–polymer analogues of amphiphilic triblock copolymers.
Nat. Mater. 6, 609–614 (2007). DOI: 10.1038/nmat1954
57
15. Brandrup, J., Immergut, E. H. & Grulke, E. A. Polymer handbook, 4th edition. (Wiley,
2004).
16. Sánchez-Iglesias, A. et al. Hydrophobic Interactions Modulate Self-Assembly of
Nanoparticles. ACS Nano 6, 11059–11065 (2012). DOI: 10.1021/nn3047605
17. Liu, K. et al. Step-Growth Polymerization of Inorganic Nanoparticles. Science. 329, 197–200
(2010). DOI: 10.1126/science.1189457
18. Lee, A. et al. Probing Dynamic Generation of Hot-Spots in Self-Assembled Chains of Gold
Nanorods by Surface-Enhanced Raman Scattering. J. Am. Chem. Soc. 133, 7563–7570 (2011).
DOI: 10.1021/ja2015179
19. Zhulina, E. B., Boulakh, A. B. & Borisov, O. V. Repulsive Forces between Spherical
Polyelectrolyte Brushes in Salt-Free Solution. Zeitschrift für Phys. Chemie 226, 625–643 (2012).
DOI: 10.1524/zpch.2012.0279
20. Kumbharkhane, A. C., Puranik, S. M. & Mehrotra, S. C. Dielectric relaxation studies of
aqueous N,N-dimethylformamide using a picosecond time domain technique. J. Solution Chem.
22, 219–229 (1993). DOI: 10.1007/bf00649245
21. Zhang, H. & Wang, D. Controlling the Growth of Charged-Nanoparticle Chains through
Interparticle Electrostatic Repulsion. Angew. Chemie Int. Ed. 47, 3984–3987 (2008). DOI:
10.1002/anie.200705537
22. Yang, M. et al. Mechanistic investigation into the spontaneous linear assembly of gold
nanospheres. Phys. Chem. Chem. Phys. 12, 11850 (2010). DOI: 10.1039/c0cp00127a
23. Liu, K., Resetco, C. & Kumacheva, E. Salt-mediated kinetics of the self-assembly of gold
nanorods end-tethered with polymer ligands. Nanoscale 4, 6574 (2012). DOI:
10.1039/c2nr31832f
24. Israelachvili, J. N. Intermolecular and Surface Forces. (Acad. Press: San Diego, CA, 2011).
58
25. Wolf, B. A. & Willms, M. M. Measured and calculated solubility of polymers in mixed
solvents: Co-nonsolvency. Die Makromol. Chemie 179, 2265–2277 (1978). DOI:
10.1002/macp.1978.021790914
26. Schulz, V. G. V. & Baumann, H. Thermodynamisches verhalten, expansionskoeffizient und
viskositätszahl von polystyrol in tetrahydrofuran. Die Makromol. Chemie 114, 122–138 (1968).
DOI: 10.1002/macp.1968.021140109
27. Franks, F., Ed. Water, A Comprehensive Treatise, Vol. 2, Ch. 7. (Plenum Press: New York,
1973)
59
Chapter 4
Surface Patterning of Nanoparticles with Polymer Patches
Portions of this chapter were adapted from: Choueiri, R.M., Galati, E., Thérien-Aubin, H.,
Klinkova, A., Larin, E.M., Querejeta-Fernández, A., Han, L., Xin, H.L., Gang, O., Zhulina, E.,
Rubinstein, M. & Kumacheva, E. Surface Patterning of Nanoparticles with Polymer Patches.
Submitted.
Contribution: R. M. Choueiri designed and carried out experiments and data analysis and
contributed to the interpretation and preparation of the manuscript.
4.1 Introduction
Surfaces functionalized with polymer brushes have many applications in sensing1,
medicine2 and anti-fouling3. A fundamental understanding of the morphology and behaviour of
polymer brushes on these substrates in different media is crucial for the rational design and
guaranteed consistent performance of these systems. It was found for brush-like polymer
molecules strongly grafted to a macroscopic planar surface that upon transfer of the polymer-
tethered substrate from a good to a poor solvent, a smooth layer segregated into micelles
composed of a dense core and stretched surface-tethered “legs”, so called “pinned” or “octopus”
micelles (Figure 4.1.1a, top).4-7. The phenomenon of polymer self-association into pinned
micelles is highly dependent on factors such as the polymer surface grafting density and the
length of the polymer5 and carries implications for polymer-modified surface whose
functionality depends on the uniformity of the polymer brush.
60
60
To this end, we extended this work into the nanoparticle domain by uniformly
functionalizing gold nanospheres with end-thiolated polymer and subjecting these nanocolloids
to poor solvency conditions for the polymer thereby inducing the formation of surface pinned
micelles on the nanospheres. We explored the effect of grafting density and polymer molecular
weight on pinned micelle formation in addition to the effect of surface curvature.
The implications of studying polymer segregation on nano-substrates are two-fold: first
we can fundamentally explore the behaviour of polymeric ligands on nanocolloids and extend
this to rough macroscopic surfaces which has far-reaching impact in the fields of antifouling8,
and biomedicine9,33, since polymeric ligands are a main source of in vivo stabilization. Studying
ligand morphology under the action of different stimuli can give insight into the unwanted
exposure of the nanoparticle core which is important for the stability of the core and
nanotoxicology10. Second, the segregation of ligands on the surface may be used to further
modify nanoparticles, providing a functional pathway to more complex architectures.
With this in mind, we hypothesized that the segregation of polymer ligands into surface-
pinned micelles with a footprint area comparable with the nanoparticle (NP) surface area can be
used as a thermodynamically mediated strategy for the patterning of high-curvature surfaces of
nanocolloids, while also providing insight into the behaviour of polymer ligands in different
media.
Patterning of colloidal particles with chemically or topographically distinct surface
domains (patches) has attracted tremendous research interest due to the unique properties of
patchy particles11-13. Surface-patterned particles act as colloidal analogues of atoms and
molecules14,15 serve as model systems in studies of phase transitions in liquid systems16, behave
as "colloidal surfactants"17 and function as templates for the synthesis of hybrid particles18.
61
61
Generation of micrometer- and submicrometer-size patchy colloids has currently reached state of
the art19-21. Surface patterning of inorganic nanoparticles with dimensions on the order of tens of
nanometers is uncommon in the colloidal domain, although they exhibit size- and shape-
dependent optical, electronic, and magnetic properties, and their assemblies show new collective
properties22.. Patterning of nanoparticles is generally achieved by their site-selective
functionalization at the liquid/liquid or liquid/solid interface, however this method is limited to
the generation of two-patch nanoparticles and requires the immiscibility of the counterpart
solvents and ligands23,24. Phase separation of low-molecular weight ligands with distinct lengths
yields nanoparticles with surface ripples25, two-patch NPs26, or in the case of polymer ligands,
nanoparticles with a “raspberry” surface morphology27. A general strategy to generate patchy
nanoparticles with different compositions and shapes would advance their applications in
fundamental research and in nanotechnologies. Here we demonstrate a new concept for
nanoparticle surface patterning, which utilizes thermodynamically driven segregation of polymer
ligands into surface-pinned micelles.
The methodology offers the ability to control the dimensions of patches, their spatial
distribution and the number of patches per nanoparticle, in agreement with a theoretical model.
The versatility of the strategy is demonstrated by patterning nanoparticles with different
dimensions, shapes and compositions, tethered with various types of polymers and subjected to
different external stimuli. This work offers a new approach to patchy nanocolloids with
applications in fundamental research, self-assembly of nanomaterials, diagnostics, sensing and
colloidal stabilization.
62
62
Figure 4.1.1 Polymer segregation on the NP surface. a, Schematics of solvent-mediated
formation of pinned polymer micelles (surface patches) on a planar macroscopic surface (top)
and on the NP surface (bottom). b-c, TEM images of gold NSs capped with PS-50K at the
grafting density of PS-50Kof 0.03 chains/nm2, which were deposited on the grid from the 0.3 nM
NP solution in DMF (b) and from the DMF-water mixture at Cw=4 vol. % after 24 h incubation
at 40 oC (c). Scale bars are 100 nm. Insets in b and c show the corresponding images of
individual NPs. Inset scale bars are 20 nm. d, Electron tomography reconstruction image of the
60 nm-diameter NP with three PS-50K patches, each shown for clarity with a different arbitrary
color. The image of a gold core is removed to highlight the structure of polymer patches. The
estimated resolution is 2-3 nm. Patchy NSs were formed as described in c. The grafting density
of PS-50K is 0.02 chains/nm2. e, TEM image of the gold NP carrying photocrosslinked thiol-
terminated polystyrene-co-polyisoprene patches preserved after 24 h incubation in
tetrahydrofuran (a good solvent for polystyrene-co-polyisoprene). Original patchy NPs were
formed after 24 h incubation in the DMF-water mixture at Cw=1 vol. %. Scale bars in d and e are
20 nm.
The proposed approach to patchy NPs is illustrated in Figure 4.1.1a (bottom). On the NP
surface, following the reduction in solvent quality, a uniformly thick polymer brush layer breaks
up into a discrete number of pinned micelles (patches) in the process that is driven by attractive
63
63
polymer-polymer interactions and the competition between the polymer grafting constraints and
the reduction in its interfacial free energy.
We validated this approach for NPs with different dimensions, shapes and chemical
compositions, which were capped with various types of polymer and copolymer ligands and
subjected to different external stimuli. We show experimentally and theoretically that the size of
patches is governed by the polymer dimensions and grafting density, while the number of
patches per NP is determined by the ratio between the NP diameter and polymer size. On
demand, the patches were permanently vitrified by polymer photocrosslinking. The resulting
patchy NPs acted as in situ colloidal surfactants and their self-assembly exhibited new binding
modalities.
4.2 Results and Discussion
To explore the proposed approach, we synthesized gold spherical NPs (nanospheres, NSs)
with a mean diameter D in the range from 20 ± 1.0 to 80 ± 1.5 nm, which were stabilized with
cetyltrimethylammonium bromide- or cetylpyridinium chloride (Please see Chapter 2 section
2.2.1.1. on page 24 for synthesis) These low-molecular weight ligands were replaced with thiol-
terminated polystyrene (PS) molecules with a molecular weight of 30,000 or 50,000 g/mol
(Figure 4.2.1–Figure 4.2.3, Table 2–Table 4). Later in the text, we refer to these polymers as PS-
30K and PS-50K, respectively. The extinction spectra of the PS-capped NSs (Figure 4.2.1)
obtained show characteristic wavelengths at the absorption peak maxima that red-shifted with
increasing NS diameter, consistent with literature28.
64
64
Figure 4.2.1. Extinction spectra of gold NSs. Normalized extinction spectra of 20 (blue), 40
(red), 60 (grey), and 80 nm-diameter (yellow) gold NSs, all stabilized with PS-50K and
dispersed in THF.
Figure 4.2.2. TEM images of gold NSs stabilized with PS-50K. Gold NSs tethered with PS-
50K and deposited on a TEM grid from the solution in THF. Average NS diameters are 20 (A),
40 (B), 60 (C), and 80 nm (D). Scale bars are 100 nm.
65
65
Figure 4.2.3. TEM images of gold NSs stabilized with PS-30K. Gold NSs capped with PS-
30K and deposited on a TEM grid from the solution in THF. Average NS diameters are 20 (A)
and 40 nm (B). Scale bars are 100 nm.
The polymer-capped NSs were dispersed in dimethylformamide (DMF), a good solvent for
PS molecules (the value of the second virial coefficient, A2, is 3.5×10−4 mol cm3 g−2, equivalent
to Flory–Huggins interaction parameter of 0.46)29. Figure 4.1.1b shows a transmission electron
microscopy (TEM) image of 20 nm-diameter NSs functionalized with PS-50K. When cast on the
grid from the solution in THF, the NSs were engulfed with a uniformly-thick polymer shell.
Following the reduction in solvent quality for the PS ligands by adding water to the NS
solution in DMF and incubation of the NSs in the mixed solvent for the time interval varying
from 1 to 24 h at 40 oC, the polymer layer transformed into a surface patch (Figure 4.1.1c). Since
the mobility of thiol-terminated ligands is suppressed for multi-facet gold NSs and for high-
molecular weight ligands, and lateral motion is generally slow (on the order of 1 nm/h)30, it can
be expected that in a poor solvent, stretched PS-50K was grafted to the NS surface, as shown in
Figure 4.1.1a, bottom. Upon polymer surface segregation, the yield of patchy NSs was 65 %;
other species included small self-assembled NS clusters (32 %) and NSs with a smooth shell (3
%). After removal of the clusters by centrifugation, the fraction of patchy NSs was 98%. Patch
66
66
formation was reversible: upon dilution of the solution with DMF to a water concentration of
Cw<1 vol. % the core-shell NS morphology was recovered.
The formation of multi-patch NSs was explored for NSs with larger dimensions. Figure
4.1.1d shows a three-dimensional electron tomography image of the 60 nm-diameter patchy gold
NS capped with PS-50K (see section 4.2.2 on page 68 for details of tomography experiments).
The NS carried three polymer patches, each shown with a different arbitrary color for clarity.
The side view, obtained from tomographic reconstruction, revealed an elongated patch shape,
which could be induced by the partial wetting of the substrate with the polymer solution. Some
accumulation of the polymer at the NS-substrate interface (Figure 4.2.5), supports this
assumption.
4.2.1 Cross-linking of patchy particles
To ensure that polymer surface segregation occurs in situ, that is in the colloidal solution,
patchy gold NSs were tethered with thiol-terminated random copolymer polystyrene-co-
polyisoprene with Mn = 53,000 g/mol. To induce copolymer surface segregation on gold 35 nm-
diameter NSs, a DMF/water solution (200 μL, Cw=2 vol. %) was added to 200 μL of the
solution of PS-co-PI-functionalized gold NSs in DMF to obtain the final concentration of water
of Cw=1 vol. %. The solution was sonicated for 5 sec and then immersed in a bath at 40 °C for
18 h to generate surface patches. We then photocrosslinked the patches formed by PS-co-PI on
the surface of gold NSs in the solution, that is, without drying them on a solid substrate. This was
achieved by adding 100 μL of the patchy NSs to 100 μL of a solution of photoinitiator
azobisisobutyronitrile (AIBN) solution (0.1 wt. %) in the DMF/water mixture at Cw=1 vol. %.
The suspension was incubated at 4 °C for 4 h to allow AIBN to diffuse into the copolymer
patches. Subsequently, the NS solution was exposed to ultra-violet illumination (UV-A lamp,
67
67
Honle, UVAPrint 40C, λ=365 nm, I=30 mW cm−2) for 5 min. Under these conditions, the AIBN
radicals initiated crosslinking of the 3,4-polyisoprene units of the PI comonomers.
To demonstrate permanent crosslinking of the PS-co-PI patches, 100 μL of the resulting
NS solution was diluted in 1 mL of THF, a good solvent for the PS-co-PI copolymer, sonicated
for 15 min and incubated at room temperature for 24 h. Figure 4.2.4A and D show that the
crosslinked patches were preserved under good solvency conditions. In the control experiments,
100 μL of the solution of patchy NSs functionalized with non-crosslinked PS-co-PI in
DMF/water mixture (Cw=1 vol. %) was diluted with 1 mL of THF, sonicated for 15 min and
incubated at room temperature for 24 h. Alternatively, the solution of patchy NSs without AIBN
was irradiated with ultra-violet light, diluted with 1 mL of THF to Cw=0.1 vol. %, sonicated for
15 min and incubated at room temperature for 24 h. In both cases, a large fraction of patchy NSs
transformed into the core-shell NSs with a smooth, uniformly-thick shell (Figure 4.2.4B and C).
Thus, we conclude that polymer patches are formed in solution, poor solvency conditions are
essential in the formation of surface patches and photocrosslinking is required to permanently
retain patches. In the rest of the text, we refer to the non-crosslinked patchy NPs, which were
characterized by analyzing their two-dimensional projections in TEM images or in electron
tomography experiments.
68
68
Figure 4.2.4 Segregation of the PS-co-PI copolymer on the surface of gold NSs.The bar
graph depicts the fraction of patchy NSs in the original DMF/water solution at Cw=1 vol. %
(grey bars) and in solution diluted with THF to Cw=0.1 vol. % (red bars). From right to left:
experiments conducted in the DMF/water solution at Cw=1 vol. % (labeled as CW1);
experiments conducted in the DMF/water solution at Cw=1 vol. % exposed to irradiation (labeled
as CW1 + UV) and experiments conducted in the DMF/water solution at Cw=1 vol. % with
added AIBN under UV irradiation (CW1 + AIBN + UV). The fraction was determined as the
average of three experiments with approximately 40 NSs counted per experiment. (B) TEM
images of PS-co-PI stabilized NSs in Cw=1 vol. % before (left) and after (right) dilution with
THF. (C) Representative TEM image of PS-co-PI stabilized NSs in Cw=1 vol. % treated with UV
irradiation in the DMF/water solution at Cw=1 vol. % before (left) and after (right) dilution with
THF. (D) TEM image of PS-co-PI stabilized NSs in the DMF/water solution at Cw=1 vol. %
subjected to UV irradiation in the presence of AIBN before (left) and after (right) dilution with
THF. The colors of the frames in (B-D) correspond to bar colors in (A). Scale bars are 20 nm.
4.2.2 Tomography experiments
The low-dose bright-field TEM tilt series imaging of patchy gold NSs was conducted on
a JEOL instrument with a LaB6 emitter operated at 120 keV from -70 to +70 degrees with two-
B
C
D
CW1 CW1 + UV CW1 +
AIBN + UV
A
Fra
ctio
n o
f p
atc
hy N
Ss (
%)
69
69
degree intervals with a total dosage of 40 electrons/Å2. The tilt images were deliberately
defocused by approximately one micrometer to increase contrast of the polymer patches. We
performed a simple contrast flipping and deliberately turned off contrast transfer correction for
image preparation for weighted back project. Due to the defocusing, all the boundaries of the
polymer patches were are outlined with a Fresnel fringe in the project images, as well as in the
reconstruction. This Fresnel contrast helped identify the boundary of the polymer patches when
hand-segmenting them by tracing the Fresnel boundary of the low-intensity polymer in one
cross-sectional image at a time.
The images were aligned using the center of mass of the NSs. The 3D data set was
reconstructed using weight back projection and 3D visualization of the reconstructions was
rendered by the Amira software. Importantly, the Fresnel fringes limited the sensitivity in
detecting an ultrathin polymer layer on the surface of the NSs. The resolution of the
reconstruction was estimated to be 2-3 nm.
Figure 4.2.5 shows the monochrome reconstructed image (top view) and 3D images of 60
nm-diameter gold NS with three PS-50K patches, each labeled with a different arbitrary color.
70
70
Figure 4.2.5 Reconstructed 3D images of a 60 nm-diameter patchy NS functionalized with
PS-50K. (A) Monochrome reconstructed image of gold NS with polymer patches (top view). (B)
3D images reconstructed from the TEM tilt series. The reconstructed gold core is removed for
clarity of visualization of the polymer patches. From left to right, the images show the rotation of
the NS around the axis normal to the plane of the TEM grid. Each patch is shown with a different
color for their visual separation. The substrate is not visible on the reconstructions, due to its low
electron density. Partial wetting of the substrate with a polymer is evident from the meniscus-like
shape of the interface between the patches and the substrate. Insets illustrate a different rotation
angle around a vertical z-axis. The orientations of the x- and y-axes are given approximately for
eye guidance. In (A and B) the scale bars are 30 nm. The NSs were incubated for 24 h in the
DMF-water mixture at Cw=4 vol. % at 40 oC.
x
yz
x
yz
xy
z
xy
zz
x
y
z
y
x
A
B
71
71
4.2.3 Polymer segregation control experiments
Control experiments on gold NSs functionalized with PS-50K were performed to
ascertain the effect of solvent quality and nanoparticle concentration on the generated patchy
nanoparticles.
4.2.3.1 Control experiments conducted in good solvent
To prove that poor solvency conditions were required to induce pinned micelle
formation, control experiments in the absence of poor solvent were performed. Briefly, in control
experiments, the 0.3 nM solution of PS-50K-capped gold NSs was maintained at 40 °C for 24 h
in DMF (good solvent) without addition of water. We note that extra care must be taken due to
the hygroscopic nature of DMF. Septa-sealed anhydrous DMF stored in a dessicator must be
used and all experimental flasks were sealed and covered with parafilm. Figure 4.2.6 shows
images of 40 and 60 nm-diameter NSs in control experiments. The maintenance of a core-shell
morphology after incubation in a good solvent confirms that a poor solvent trigger is required for
the formation of surface pinned micelles.
72
72
Figure 4.2.6 Polymer-capped gold NSs in a good solvent. TEM images of 40 nm (A) and 60
nm (B) diameter gold NSs stabilized with PS-50K and incubated in DMF for 24 h at 40 oC. Scale
bars are 100 nm. Insets show corresponding individual NSs. Scale bars are 40 nm.
4.2.3.2 Control experiments performed at high water content.
To ascertain whether an increase in the concentration of poor solvent would qualitatively
affect the morphology of the generated polymer patches, polymer surface segregation
experiments were conducted for gold NSs in the DMF/water mixture at Cw=10 vol. %, that is,
water content was 2.5-fold higher than in the standard polymer segregation experiments
described. A 0.3 nM solution of 20 nm-diameter gold NSs in 500 μL of DMF was sonicated for 5
s. Then, 500 μL of a DMF/water mixture at Cw=20 vol. % was added dropwise to the NS
solution in DMF under gentle swirling of the vial, to reach the resulting total water concentration
Cw=10 vol. % (as opposed to 4 vol. % in typical experiments). The vial was sealed and
maintained in a water bath at 40 °C for 24 h. Figure 4.2.7 shows that increase in water content in
the DMF/water mixture did not qualitatively change the formation of patchy NSs.
A B
73
73
Figure 4.2.7 Polymer-capped gold NSs in the DMF/water mixture at Cw =10 vol. %. TEM
images of gold NSs stabilized with PS-50K. The NSs at the concentration of 0.3 nM were
incubated in the DMF-water mixture at Cw=10 vol. % for 24 h at 40 oC. Scale bars are 100 nm.
Insets show representative single-patch (right) and two-patch (left) gold NSs. Scale bars are 50
nm.
4.2.3.3 Control experiments conducted at high NS concentration.
To maximize the number of individual (that is, non-assembled) patchy NSs, we conducted
polymer surface segregation experiments using very low nanoparticle concentrations of 0.3 nM
in the solution to suppress their self-assembly. To illustrate the effect of NS concentration on the
self-assembly, polymer surface segregation experiments were conducted for gold NSs in the
DMF/water mixture at Cw=4 vol. % and NS concentration of 3 nM, that is, ten-fold higher than
that described in the standard polymer segregation experiments. A 3 nM solution of 20 nm-
diameter gold NSs in 500 μL of DMF was sonicated for 5 s. Then, 500 μL of a DMF/water
mixture at Cw=8 vol. % was added dropwise to the NS solution in DMF under gentle swirling of
the vial, to reach the resulting total concentration of water Cw=4 vol. %. The vial was sealed and
maintained in a water bath at 40 ° C for 24 h. Figure 4.2.8 shows representative TEM images
obtained after heating and a histogram of species obtained as determined by TEM imaging. The
74
74
phase-segregation of PS-50K on the NS surface was qualitatively similar to that occurring in
dilute NS solutions, however a larger fraction of NSs formed clusters due to the physical bonding
of polymer ligands grafted to different NSs, confirming that dilute solutions of NSs are required
to maximize the yield of individual patchy NSs.
Figure 4.2.8 TEM images of polystyrene-stabilized gold NSs (3.0 nM concentration) in the
DMF/water mixture. (A, B) TEM images of 20 nm-diameter gold NSs stabilized with PS-50K.
The NSs were incubated in the DMF-water mixture at Cw=10 vol. % for 24 h at 40 oC. The
concentration of NSs in the solution was 3 nM. Scale bars are 200 nm. (C) Average fraction of
individual NSs determined in polymer phase segregation experiments at NS concentration of 0.3
nM (blue) and 3.0 nM (green), based on three series of experiments. In each experiment, 100
species were analyzed. Error bars represent the standard deviation.
4.2.4 Effect of nanosphere diameter on patch formation
Patch formation and their structure on the NS surface was governed by polymer length, NS
diameter and polymer grafting density. In the first series of experiments, we examined transitions
0
20
40
60
80
100
3
Frac
tio
n o
f in
div
idu
al N
Ss ,
%
A
B
C
Concentration of NSs (nM)
0.3 3.0
75
75
between the NSs with a smooth polymer shell (core-shell NSs) and patchy NSs at varying ratios
between the NS and polymer size. The NSs with a diameter 20D80 nm were capped with PS-
30K or PS-50K having the molecular radius, R, of 11 or 15 nm, respectively31,32. At a constant
grafting density of polymer chains with a radius R, polymer segregation was favored for small
NSs (Figure 4.2.9a, top and Figure 4.2.10–Figure 4.2.14), while the reduction in R led to a larger
number of patches per NS at constant D and (Figure 4.2.9a, bottom). Figure 4.2.9b illustrates
these trends for varying D/R ratios (characterizing a different extent of stretching of the micellar
"legs"). For example, at D/R=1.3 (D=20 nm, PS-50K), 98% of NSs had a single patch, while at
D/R=2.6 (D=34 nm, PS-50K), 34 and 53 % of the NSs had one and two patches, respectively.
Figure 4.2.9. Effect of NS curvature to polymer size ratio on surface patch formation. a,
Effect of NS size (top) and polymer dimensions (bottom) on patch formation. The NSs are
functionalized with PS-50K (top and left bottom) and PS-30K (bottom right) at =0.03
chains/nm2. Scale bars are 25 nm. b, Distribution of populations of NSs with a different patch
number. The red, yellow, blue and violet bars correspond to the 20, 40, 60 and 80-nm-diameter
NSs capped with PS-50K, respectively, the green bar represents 32 nm-diameter NSs
functionalized with PS-30K. =0.03 chains/nm2. The error bars represent the standard
deviations. The inset shows the D/R ratios, with colors corresponding to the colors of bars and
the frames of the images in a. In b, 200-300 NSs were analyzed for each NP population.
76
76
The angles between the patch centers were 170±9o and 120±13o for two-patch and three-
patch NSs, respectively, characterizing the uniformity of patch distribution on the NS surface.
The average maximum patch height increased with polymer molecular weight: for two-patch 40
nm-size NSs capped with PS-50K and PS-30K it was 9.0±0.31 and 6.5±0.65 nm, respectively.
Figure 4.2.10–Figure 4.2.14 show patches of PS-50K on the surface of 20, 40, 60, and 80 nm-
diameter gold NSs. Figure 4.2.12 shows surface patches of PS-30K on 40 nm-diameter gold NSs.
Figure 4.2.10. Polymer segregation on the surface of gold NSs. TEM image of 20 nm-
diameter gold NSs stabilized with PS-50K. The NSs were incubated in the DMF-water mixture
at Cw=4 vol. % for 24 h at 40 oC. Scale bar is 500 nm. Inset: Representative image of single-
patch NS. Scale bar is 50 nm.
77
77
Figure 4.2.11. Polymer segregation on the surface of gold NSs. TEM images of 40 nm-
diameter gold NSs stabilized with PS-50K. The NSs were incubated in the DMF-water mixture
at Cw=4 vol. % for 24 h at 40 oC. Scale bars are 500 nm. Insets show representative single-patch
(top) and two-patch (bottom) NSs. Inset scale bars are 50 nm.
Figure 4.2.12. Polymer segregation on the surface of gold NSs. TEM images of 40 nm-
diameter gold NSs stabilized with PS-30K. The NSs were incubated in the DMF-water mixture
A B
78
78
at Cw=4 vol. % for 24 h at 40 oC. Scale bars are 100 nm. Insets show representative three-patch
(left) and two-patch (right) NSs. Scale bars are 50 nm.
Figure 4.2.13. Effect of grafting density on polymer segregation on the NS surface. (A, B)
TEM image of 60 nm-diameter gold NSs stabilized with PS-50K with a grafting density of 0.03
(A) and 0.02 (B) chains/nm2, both incubated for 24 h at 40 oC in the DMF-water mixture at Cw=
4 vol. %. Scale bars are 100 nm. Inset: scale bars are 50 nm.
Figure 4.2.14. Effect of grafting density on polymer segregation on the surface of NSs. (A,
B) TEM image of 80 nm-diameter gold NSs stabilized with PS-50K with a grafting density of
0.03 (A) and 0.012 (B) chains/nm2, both in incubated for 24 h at 40 oC in the DMF-water
mixture at Cw=4 vol. %. Scale bars are 100 nm. Inset: scale bars are 50 nm.
A B
79
79
4.2.5 Effect of polymer grafting density on patch formation
Next, the formation of patches was examined for varying grafting density, , of PS-50K
capping NSs with different dimensions. Figure 4.2.15a shows the experimental diagram of NS
states, plotted in the D- parameter space. The transition between the core-shell and patchy NSs
was favored at decreasing and D (or increasing curvature) values, signified by the negative
slope of the boundary solid blue line.
In the patchy region, the average number of patches per NS, n, increased with the NS
diameter and did not noticeably vary with (and was thus averaged over the range of studied).
The latter effect further supports the lack of lateral mobility of thiol-terminated polymer ligands
on the NS surface. The polymer grafting density influenced patch dimensions. For example, for
three-patch NSs with D=60 nm the average maximum height of the PS-50K patch decreased
from 7.7±1.1 to 3.1±0.35 nm when reduced from 0.02 to 0.003 chains/nm2, respectively.
Figure 4.2.15. Structural transitions in the polymer layer on the surface of gold NSs.a,
Experimental diagram of NS states. The blue line separates the regions of core-shell and patchy
NSs with a different average patch number n. The insets illustrate a patchy and a core-shell NSs
with of 0.012 and 0.03 chains/nm2, respectively. Scale bars are 50 nm. In a, 200-300 NSs were
80
80
analyzed for each NP population. b, Theoretical diagram of NS states. Red lines show transitions
between the NSs with different values of n. The transitions begin at DR (indicated on the
vertical axis); the expression /(bR) (upper horizontal axis) designates σ corresponding to the
transition between the smooth and patchy polymer layers on planar surfaces or large NSs.
The trends shown in Figure 4.2.15a were captured in the theoretical state diagram in Figure
4.2.15b (the theoretical model is described in more detail in Appendix 2). The structure of the
polymer layer on the NS surface was governed by the polymer-solvent interfacial energy and the
energy of stretching of end-tethered polymer molecules. In Figure 4.2.15b, at high σ values (the
right region of the diagram), extended polymer chains minimized their interfacial and stretching
energies by forming a smooth layer31.
For large NSs, the transition between the two regions is shown as a blue line approaching
the grafting density /(bR), where τ accounts for the solvent quality and b is the monomer length.
At σ = /(bR), the layer thickness is comparable to an unperturbed polymer size R. For large
NSs, at a lower σ, the polymer molecules in the layer are no longer extended. To reduce the
interfacial energy, the condensed polymer phase undergoes de-wetting, that is, the segregation of
the smooth layer into small droplets (corresponding to the cores of pinned micelles). This effect
results in the exposure to the solvent of the NS surface with a lower NS-solvent interfacial
tension γ1 than the tension γ2 ≈ kTτ2/b2 of the polymer-solvent interface29. The de-wetting is
constrained, because the polymer molecules (micellar “legs”) are strongly surface-tethered and
have to stretch to reach micellar cores. At lower values of (the left region of the diagram), the
layer became thinner than the unperturbed molecular size of the polymer and the interfacial
polymer-solvent energy was lowered by polymer segregation in pinned micelles. The elastic
energy of stretched micellar “legs” was comparable to the polymer-solvent interfacial energy.
81
81
The competition between the interfacial and stretching energies resulted in the optimized
micelle footprint area A~(N2τ/σ)2/5, where N is the polymer degree of polymerization. Since the
number of micelles per NS is proportional to the ratio between the NS surface area, D2, and the
micelle footprint area A, for varying NS dimensions and/or polymer grafting densities, transitions
were expected between the NSs with n and n+1 patches. The inclined red lines with constant
D2/A ratios in Figure 4.2.15b outline these transitions, with single-patch (n=1) NSs at the
bottom and a transition between n=1 and n>1 at D~R.
The effect of NS size (or surface curvature) on patch formation was revealed by the
position and incline of the boundary between the core-shell and patchy NS states. The balance
between the interfacial energy of the polymer and the free energy of stretching of the micellar
“legs” led to a higher stability of micelles on small NSs and hence a negative slope of the
boundary line. Thus overall, the experimental and theoretical results were in excellent
agreement.
4.3 Conclusions
In summary, we developed a new strategy for nanoparticle surface patterning that is
governed by thermodynamically controlled segregation of polymer ligands in pinned micelles
with a footprint area comparable with the nanoparticle surface area. This effect is favored for
small nanoparticles with a large surface curvature. The experimental results were in excellent
agreement with the proposed theoretical model. The described patterning strategy can be used for
the generation of reconfigurable nanocolloids: reversible transitions between a smooth polymer
shell and surface patches can be triggered by illumination, change in temperature, ionic strength
or pH of the solution, that is, the stimuli changing the solvent quality. On demand, polymer
patches can be "locked" by permanent crosslinking, which would suppress NP assembly and
82
82
enable the utilization of solutions with a higher NP concentration, thereby increasing the yield of
patchy NPs.
The utilization of block copolymers will facilitate NP patterning with a variety of pinned
micelle structures, including co-micelles, which may tailor new functionalities to patchy NPs.
"Grafting-from" functionalization and fractionation of NPs with a particular number of patches
will enhance control over the number of patches per NP. Patterning of multicomponent NPs and
the self-assembly of patterned NPs into complex, hierarchical structures are other directions to
explore. Furthermore, given the remarkable current progress in the synthesis of NPs with
different shapes, the proposed strategy enables fundamental studies of polymer segregation on
surfaces with large curvatures or surfaces with multiple curvatures.
We note that in addition to the generation of patchy NPs, polymer segregation on the
surface of NPs has other far-reaching implications. Polymer-tethered NPs have a broad range of
applications in imaging and medical diagnostics33 , therapeutics34, and chemical sensing35.
83
83
4.4 References
1. Chen, T., Ferris, R., Zhang, J., Ducker, R. & Zauscher, S. Stimulus-responsive polymer
brushes on surfaces: Transduction mechanisms and applications. Prog. Polym. Sci. 35, 94–112
(2010). DOI: 10.1016/j.progpolymsci.2009.11.004
2. Stuart, M. A. C. et al. Emerging applications of stimuli-responsive polymer materials. Nat.
Mater. 9, 101–113 (2010). DOI: 10.1038/nmat2614
3. Yu, K., Mei, Y., Hadjesfandiari, N. & Kizhakkedathu, J. N. Engineering biomaterials surfaces
to modulate the host response. Colloids Surfaces B Biointerfaces 124, 69–79 (2014). DOI:
10.1016/j.colsurfb.2014.08.009
4. Williams, D. R. M. Grafted polymers in bad solvents: octopus surface micelles. J. Phys. II 3,
1313 (1993). DOI: 10.1051/jp2:1993202
5. Zhulina, E. B., Birshtein, T. M., Priamitsyn, V. A. & Klushin, L. I. Inhomogeneous structure
of collapsed polymer brushes under deformation. Macromolecules 28, 8612 (1995). DOI:
10.1021/ma00129a021
6. Koutsos, V., van der Vegte, E. W., Pelletier, E., Stamouli, A. & Hadziioannou, G. Structure of
chemically end-grafted polymer chains studied by scanning force microscopy in bad-solvent
conditions. Macromolecules 30, 4719 (1997). DOI: 10.1021/ma961625d
7. Choi, B.C., Choi, S. & Leckband, D. E. Poly(N-isopropyl acrylamide) brush topography:
dependence on grafting conditions and temperature. Langmuir 29, 5841 (2013). DOI:
10.1021/la400066d
8. Banerjee, I., Pangule, R. C. & Kane, R. S. Antifouling Coatings: Recent Developments in the
Design of Surfaces That Prevent Fouling by Proteins, Bacteria, and Marine Organisms. Adv.
Mater. 23, 690–718 (2011). DOI: 10.1002/adma.201001215
9 Smith, J. R. & Lamprou, D. A. Polymer coatings for biomedical applications: a review. Trans.
IMF 92, 9–19 (2014). DOI: 10.1179/0020296713z.000000000157
84
10. Van Hoecke, K. et al. Ecotoxicity and uptake of polymer coated gold nanoparticles.
Nanotoxicology 7, 37–47 (2013). DOI: 10.3109/17435390.2011.626566
11. Bianchi, E., Blaak, R. & Likos, C. N. Patchy colloids: state of the art and perspectives. Phys.
Chem. Chem. Phys. 13, 6397 (2011). DOI: 10.1039/C0CP02296A
12. Preisler, Z., Vissers, T., Munaŏ, G., Smallenburg, F. & Sciortino, F. Equilibrium phases of
one-patch colloids with short-range attractions. Soft Matt. 10, 5121 (2014). DOI:
10.1039/C4SM00505H
13. Chen, Q., Bae, S. C. & Granick, S. Directed self-assembly of a colloidal kagome lattice.
Nature 469, 381 (2011). DOI: 10.1038/nature09713
14. Glotzer, S. C., & Solomon, M. J. Anisotropy of building blocks and their assembly into
complex structures. Nature Mater. 6, 557 (2007). DOI: 10.1038/nmat1949
15. Groschel, A. H. et al. Guided hierarchical co-assembly of soft patchy nanoparticles. Nature
503, 247–251 (2013). DOI: 10.1038/nature12610
16. Kern, N. & Frenkel, D. Fluid–fluid coexistence in colloidal systems with short-ranged
strongly directional attraction. J. Chem. Phys. 118, 9882 (2003). DOI: 10.1063/1.1569473
17. Binks, B. P. Particles as surfactants: similarities and differences. Curr. Opin. Coll. Interface
Sci. 7, 21 (2002). DOI: 10.1016/S1359-0294(02)00008-0
18. Chen, T., Chen, G., Xing, S., Wu, T. & Chen, H. Scalable routes to Janus Au-SiO2 and
ternary Ag-Au-SiO2 nanoparticles. Chem. Mater. 22, 3826–3828 (2010). DOI:
10.1021/cm101155v
19. Wang, Y. et al. Colloids with valence and specific directional bonding. Nature 491, 51
(2012). DOI: 10.1038/nature11564
20. Pawar, A. B. & Kretzschmar, I. Fabrication, assembly, and application of patchy particles.
Macromol. Rapid Comm. 31, 150. (2010). DOI: 10.1002/marc.200900614
85
21. Sacanna, S. & Pine, D. J. Shape-anisotropic colloids: building blocks for complex
assemblies. Curr. Opin. Colloid Interface Sci. 16, 96 (2011). DOI: 10.1016/j.cocis.2011.01.003
22. Nie, Z.H., Petukhova, A., & Kumacheva, E. Properties and emerging applications of self-
assembled structures of inorganic nanoparticles. Nature Nanotech. 5, 15 (2010). DOI:
10.1038/nnano.2009.453
23. Lattuada, M. & Hatton, T. A. Synthesis, properties and applications of Janus nanoparticles.
Nano Today 6, 286 (2011). DOI: 10.1016/j.nantod.2011.04.008
24. Andala, D. M., Shin, S. H. R., Lee, H. Y. & Bishop, K. J. M. Templated synthesis of
amphiphilic nanoparticles at the liquid-liquid interface. ACS Nano 6, 1044 (2012). DOI:
10.1021/nn202556b
25. Jackson, A. M., Myerson, J. W. & Stellacci, F. Spontaneous assembly of subnanometre
ordered domains in the ligand shell of monolayer-protected nanoparticles. Nature Mater. 3, 330
(2004). DOI: 10.1038/nmat1116
26. Vilain, C., Goettmann, F., Moores, A., Le Floch, P. & Sanchez, C. Study of metal
nanoparticles stabilised by mixed ligand shell: a striking blue shift of the surface-plasmon band
evidencing the formation of Janus nanoparticles. J. Mater. Chem. 17, 3509 (2007). DOI:
10.1039/b706613a
27. Bao, C.et al. Effect of molecular weight on lateral microphase separation of mixed
homopolymer brushes grafted on silica particles. Macromolecules 47, 6824 (2014). DOI:
10.1021/ma501474m
28. Gold Nanoparticles: Properties and Applications (Sigma-Aldrich technical report;
http://www.sigmaaldrich.com/materials-science/nanomaterials/gold-nanoparticles.html).
29. Wolf, B. A. & Willms, M. M. Measured and calculated solubility of polymers in mixed-
solvents-co-non-solvency. Makromol. Chem. 179, 2265 (1978). DOI:
10.1002/macp.1978.021790914
30. Bürgi, T. Properties of the gold–sulphur interface: from self-assembled monolayers to
clusters. Nanoscale 7, 15553 (2015). DOI: 10.1039/C5NR03497C
86
31. Rubinstein, M. & Colby, R. H. Polymer Physics. Oxford University Press, 2003.
32. The unperturbed chain size is polymer brushes is typically defined as the root-mean-square
end-to-end distance R of a polymer in its ideal conformation, which we call “chain radius R” in
the paper.
33. Erathodiyil, N. & Ying, J. Y. Functionalization of inorganic nanoparticles for bioimaging
applications. Acc. Chem. Res. 44, 925 (2011). DOI : 10.1021/ar2000327
34. Minelli, C., Lowe, S. B. & M. M. Stevens, Engineering nanocomposite materials for cancer
therapy. Small 6, 2336 (2010). DOI: 10.1002/smll.201000523
35. Saha, K., Agasti, S. S., Kim, C., Li, X. & Rotello, V. M. Gold nanoparticles in chemical and
biological sensing. Chem. Rev. 112, 2739 (2012). DOI: 10.1021/cr2001178
87
88
88
Chapter 5
Exploring the Versatility and Applications of the Nanopatterning
Method
Portions of this chapter were adapted from: Choueiri, R.M., Galati, E., Thérien-Aubin, H.,
Klinkova, A., Larin, E.M., Querejeta-Fernández, A., Han, L., Xin, H.L., Gang, O., Zhulina, E.,
Rubinstein, M. & Kumacheva, E. Surface Patterning of Nanoparticles with Polymer Patches.
Accepted.
Contribution: R. M. Choueiri designed and carried out experiments with gold spherocylindrical
nanorods, and gold nanorods with a dumbbell shape, performed data analysis, self-assembly
experiments in the presence of excess polymer and contributed to the interpretation and
preparation of the manuscript.
5.1 Introduction
Further to the work described in Chapter 4 for the surface segregation of polymers on
gold nanospheres (NSs), the versatility of the polymer segregation-induced nanopatterning
technique was explored for gold spherocylindrical nanorods (NRs), gold nanorods with a
dumbbell shape (NDs), gold nanocubes (gold NCs) and silver nanocubes (silver NCs) stabilized
by polystyrene capping ligands. Our aim in studying the polymer segregation on NPs with
different shapes was to examine how pinned micelles form on NPs with different and complex
curvature profiles, including NPs combining planar and highly curved regions, as well as regions
with a negative curvature.
89
89
In addition, our objective was to explore the versatility of the polymer segregation
technique with regards to different ligands including pH-responsive polymers ( e.g. poly(4-vinyl
pyridine) and conductive polymers (e.g. poly(N-vinyl carbazole)). We specifically chose
polymers with high electron density, that is, visible by transmission electron microscopy, in
order to easily assess morphological changes in polymer surface layers. These polymer systems
validate the versatility of the nanopatterning method which can be considered as a general
approach towards patchy colloids. Our aim in exploring different polymer ligands was to open
the door for a wide variety of polymer surface segregation systems, which may be more relevant
in different chemical and biological contexts.
Lastly, the self-assembly of patchy NSs and patchy NCs in a poor solvent for the capping
ligands was explored to pave the way for the new self-assembly modes accessible to particles
with surface patches. The self-assembly behaviour of patchy NSs was also explored in the
presence of free polymer in a poor solvent, demonstrating the use of patchy NPs as colloidal
surfactants.
5.2 Results and Discussion
5.2.1 Polymer segregation on nanoparticles with different shapes
The versatility of the nanopatterning concept was explored for NPs with different shapes
and compositions, capped with different polymer ligands and subjected to different solvents.
Following the prediction of the theoretical model on a stronger tendency of patch formation on
surfaces with a high curvature, we examined polymer segregation on NRs, NCs and NDs. NRs,
NDs and NCs were all synthesized by aqueous surfactant-mediated syntheses described in
Chapter 2 (sections 2.2.1.2 – 2.2.1.5) Electron microscopy images of the as-synthesized particles
drop-cast from water and dried are presented in Figure 5.2.1 – Figure 5.2.4. Ligand exchange
90
90
was conducted to replace the surfactant on the NP surface with PS-50K via addition of the
concentrated nanoparticle solution to a solution of PS-50K in THF (as described in Chapter 2
section 2.2.1.6). Electron microscopy characterization of NRs (Figure 5.2.1), NDs (Figure 5.2.2),
silver NCs (Figure 5.2.3) and gold NCs (Figure 5.2.4) was performed on both surfactant-coated
and polymer coated NPs. These images show that NPs functionalized with polymer exhibited a
diffuse uniform polystyrene shell on their surface.
Figure 5.2.1. Electron microscopy images of gold NRs. SEM image of gold NRs stabilized
with CTAB and deposited on a TEM grid from the solution in water. Scale bar is 100 nm. Inset:
TEM image of gold NRs stabilized with PS-50K, following ligand exchange. Scale bar is 50 nm.
91
91
Figure 5.2.2. TEM images of gold NDs. TEM image of gold NDs stabilized with CTAB and
deposited on a TEM grid from the solution in water. Scale bar is 100 nm. Inset: TEM image of
gold ND stabilized with PS-50K, following ligand exchange procedure. Scale bar is 50 nm.
Figure 5.2.3. TEM images of silver NCs. TEM image of silver NCs stabilized by CTAB and
deposited on a TEM grid from the solution in water. Scale bar is 100 nm. Inset: TEM image of
silver NC stabilized with PS-50K, following ligand exchange procedure. Scale bar is 20 nm.
92
92
Figure 5.2.4. TEM images of gold NCs. TEM image of gold NCs stabilized by CTAB and
deposited on a TEM grid from the solution in water. Scale bar is 100 nm. Inset: TEM image of
gold NC stabilized with PS-50K, following ligand exchange procedure. Scale bar is 20 nm.
To achieve PS-50K surface segregation on the surface of the NPs with different shapes, the
solvent was evaporated from 100 μL of the as-prepared solution of PS-50K-capped silver NCs,
gold NCs, gold NRs or gold NDs in THF. The NPs were redispersed in 250 μL of DMF and
sonicated for 5 s. Then, 250 μL of a DMF/water mixture at Cw=8 vol. % was added dropwise to
the NP solution in DMF under gentle swirling of the vial, to reach a resulting total water
concentration of Cw= 4 vol. %. The vial was sealed and maintained at 40 ° C for 24 h (12 h for
silver NCs). High magnification images of representative morphologies of NPs with different
shapes displaying polymer surface segregation shown in Figure 5.2.5 attest to the preference for
patches to form on high curvature regions
Following incubation of PS-50K-capped spherocylindrical gold NRs in the DMF-water
mixture, a uniform polymer layer showed a trend for the separation into two distinct patches
engulfing NR tips (Figure 5.2.5a, Figure 5.2.6). The NR dimensions were 16 ± 1 nm × 80 ± 7
nm, with the width being comparable to 20 nm diameter nanospheres studied in Chapter 4
93
93
indicating a tendency for singular patch formation at the rounded ends. The long edge may be
considered comparable to two 40 nm diameter nanospheres however, the segregation behaviour
is affected by the cylindrical shape.
We quantified the tendency for patchy NRs to form by statistical analysis of TEM images.
The images of three separate experiments were analyzed, with 100 species characterized in each
experiment. It was found that the majority of NRs displayed some form of surface segregation of
the polymer ligands (Figure 5.2.6f).
Figure 5.2.5. Generality of polymer patterning of NP surface. a-c, TEM images of PS-50K-
coated gold spherocylindrical (a) and dumbbell-shaped NRs (b), silver NCs (c) and triangular
prisms (d), all in the DMF/water mixture at Cw=4 vol. %. Scale bars in a-d are 40 nm.
Similar polymer segregation toward metal tips occurred for gold nanorods with a dumbbell
shape (Figure 5.2.5b, Figure 5.2.7). NDs have regions of both positive curvature (at the tips) and
negative curvature (near the mid-point of the particle). The ND tips had a diameter of 34 ± 2 nm
94
94
comparable to 40 nm diameter NSs studied in Chapter 4. It was observed that some NDs display
multiple patches on the ND tips consistent with our previous findings (Figure 5.2.7).
Break up of PS-50K in patches occurred on the surface of silver nanocubes (Figure 5.2.5c,
Figure 5.2.8), gold nanocubes (Figure 5.2.9) and triangular nanoprisms incubated in a poor
solvent (Figure 5.2.5d).
Thus we conclude that the dominant preference for polymer patches on shaped NPs
appears to be regions of high positive curvature such as the edges and vertices of NCs and the
rounded ends of NDs and NRs. In the case of NRs, the variety of polymer segregation
morphologies visible suggest a complex effect of the cylindrical portion of the NR on polymer
segregation including the possibility of forming spirals on the NR substrate (Figure 5.2.6b).
Work is under way to explore the segregation of polymers on longer NRs to see if the formation
of extended spirals is controllable.
95
95
Figure 5.2.6. Polymer segregation on the surface of gold NRs. (a-d) TEM images of sphero-
cylindrical gold NRs stabilized with PS-50K and incubated in the DMF-water mixture at Cw= 4
vol. % for 24 h at 40 oC. Scale bar is 100 nm. (e) TEM image of gold NRs shown in A-D. Scale
bar is 500 nm. (f) Bar graph of the fractions of gold NRs with a uniformly-thick and surface-
segregated PS layers. The data in (f) were collected from three experiments for 100 species in
each experiment. Error bars represent the standard deviation.
96
96
Figure 5.2.7. Polymer segregation on the surface of gold NDs. TEM image of gold NDs
stabilized with PS-50K and incubated in the DMF-water mixture at Cw=4 vol. % for 24 h at 40
oC. Scale bar is 100 nm. Inset shows a representative image of an individual gold ND. Scale bar
is 50 nm.
Figure 5.2.8. Polymer segregation on the surface of silver NCs. TEM images of silver NCs
stabilized with PS-50K and incubated in the DMF-water mixture at Cw=4 vol. % for 12 h at 40
oC. Inset shows an individual silver NC. Scale bars are 50 nm.
97
97
Figure 5.2.9. Polymer segregation on the surface of gold NCs. TEM image of gold NCs
stabilized with PS-50K and incubated in the DMF-water mixture at Cw=4 vol. % for 24 h at 40
oC. Scale bar is 100 nm. Inset shows an individual gold NC. Scale bar is 50 nm.
5.2.1.1 Polymer segregation in THF-water mixtures
To show the versatility of the polymer segregation method with respect to solvent
composition, PS-50K segregation in solvent mixtures other than DMF-water mixtures was
explored. Experiments conducted in THF-water mixtures for gold NCs yielded a qualitatively
similar result in polymer surface segregation (Figure 5.2.10). Briefly, 250 μL of a THF/water
mixture at Cw=8 vol. % was added dropwise to 250 μL of the solution of PS-50K-capped gold
NCs in THF under gentle swirling of the vial, to reach the resulting Cw=4 vol. %. The vial was
sealed and maintained at 40 °C for 24 h. Figure 5.2.10a and b show TEM images of patchy NCs
formed under conditions described above and PS-50K-stabilized NCs after 24 h incubation in
THF at 40 ° C (control experiment).The preferred formation of polymer patches on the ribs of
NCs, that is, on high-curvature geometric features of these NPs was in agreement with the
theoretical model predicting a lower interfacial energy for pinned micelles formed on small NPs
with a large curvature. These results indicate that different solvent compositions may be used to
achieve nanopatterning as long as the conditions for triggering polymer segregation are met.
98
98
Figure 5.2.10. Polymer segregation on the surface of gold NCs in the THF-water mixture.
(a, b) TEM images of gold NCs stabilized with PS-50K and incubated for 24 h at 40 oC in the
THF-water mixture at Cw=4 vol. % (a) and in THF (control experiment) (b). Scale bars are 25
nm.
5.2.2 Surface segregation of different polymer ligands
The versatility of the pinned micelle approach to NP patterning was explored for NSs
capped with thiol-terminated PS, poly(4-vinyl pyridine) and poly(N-vinyl carbazole). All
polymer ligands exhibited qualitatively similar surface segregation in a poor solvent (Chapter 4).
For instance, thiol-terminated poly(4-vinyl pyridine) (Mn= 22,000 g/mol) on gold NSs formed
into a patch following an increase in pH of an aqueous NS solution from 2.5 to 11.5, at which
the polymer became hydrophobic (Figure 5.2.5e). To achieve this, 100 μL of the solution of
poly(4-vinyl pyridine)-functionalized gold NSs in 0.01 M HCl was added to 900 μL of NaOH
(0.1 M). The final pH of the solution was 10.5. The solution was incubated at 40 °C for 24 h.
Figure 5.2.11B shows TEM images of the controlled system at pH=2.5 (left) and a patchy NS at
pH=10.5 (right).
99
99
Figure 5.2.11. Surface segregation of polymer on the surface of gold NSs. (a) TEM image of
a poly(4-vinyl pyridine)-capped gold NS deposited from the aqueous acidic solution (pH=2.5), a
good solvent for poly(4-vinyl pyridine) (left) and the aqueous basic solution (pH=10.5), a poor
solvent for poly(4-vinyl pyridine) (right). (b) TEM image of a poly(N-vinyl carbazole)-capped
gold NS deposited from DMF (left) and the DMF-water mixture at Cw=2 vol. % after 24 h
incubation at 40 ° C (right).
Similarly, poly(N-vinyl carbazole) ligands (Mn=19,800 g/mol) segregated into two patches
on the surface of gold NSs incubated in the DMF-water mixture (Figure 5.2.5f). Briefly, 25 μL
of the solution of poly(N-vinyl carbazole-functionalized NSs in THF was dried and redispersed
with 125 μL DMF. The solution was sonicated for ~5s before dropwise addition of 125 μL of a
DMF-water mixture with Cw=4 vol. %. The final solution was incubated at 40 ° C for 24 h.
Figure 5.2.11A shows TEM images of poly(N-vinyl carbazole)-tethered NSs in the solution in
DMF (control system) (left) and the NS displaying polymer surface segregation in the DMF-
water mixture (right).
100
100
5.2.3 Self-assembly of single-patch nanospheres
The generation of patchy NPs enabled preliminary exploration of their new self-assembly
modalities. In the experiments on polymer surface segregation, we suppressed NP self-assembly
in a poor solvent by using dilute solutions, that is, by reducing the probability of NP collisions.
However, given sufficient time patchy NSs assembled in chains co-existing with small clusters
of 2-3 single-patch NSs. The self-assembly of gold NSs carrying a single patch of PS-50K
(Figure 5.2.12a) was examined after their 15 day incubation in the DMF-water mixture at Cw=4
vol. % at 40 ºC. Isolation of small NS clusters from predominant NS chains was performed by 10
min-long centrifugation of 1.5 mL of the NS solution at 4,000 g. After centrifugation, the top
0.75 mL of the solution was separated from the bottom 0.75 mL above the sediment.
Representative images of the top and bottom fractions collected after centrifugation are
shown in Figure 5.2.12b and c, respectively. The top fraction contained small NS clusters,
namely, dimers and trimers, in which the NSs were linked via a PS patch (Figure 5.2.12b). A
network of NS chains imaged prior to centrifugation is shown in Figure 5.2.12d. Importantly,
dimer and trimer building blocks (highlighted with red and yellow circles, respectively) could be
clearly discerned in the short chains and in the network, which suggested two time scales in NS
assembly (Figure 5.2.12c and d), suggesting a new, two-step mechanism of NP assembly. First,
the patchy NSs assembled in dimers and trimers, due to the efficient binding of PS-50K patches.
Then, the dimer and trimer intermediate building blocks assembled into chains, due to the less
efficient binding between the NSs surfaces deprived of the polymer.
Inspection of isolated chains of patchy NSs revealed that they were built from dimers and
trimers (Figure 5.2.12c and d), suggesting a sequential mechanism of the self-assembly of patchy
101
101
NSs: a faster assembly of dimers and trimers and a slower assembly of these building blocks in
chains, in comparison with self-assembly of non-patchy NSs1.
Figure 5.2.12. Self-assembly of single-patch gold nanospheres. a, SEM image of single-patch
gold NSs. b-d, TEM images of small clusters (b), short chains (c) and linear and network
structures (d). In (c, d) several dimer and trimer building blocks are highlighted with red and
yellow circles, respectively. The structures shown in (b, c) were separated by 10 min
centrifugation of 1.5 mL of the NS solution at 4,000 g and separation of the top solution (b) and
the bottom-level solution (c). The sample in (d) is imaged before centrifugation. The NSs were
102
102
capped with PS-50K and incubated in the DMF-water mixture at Cw=4 vol. % at 40 ºC for 15
days. The images are representative of at least 20 inspected images. Scale bars are 100 nm.
The robustness of the self-assembled chains was examined by 45 min sonication of the
solution of patchy NS after its 15 day incubation. Figure 5.2.13 shows a representative dark field
mode SEM image of the sonicated chains. Inspection of ten SEM images revealed that the NS
chains did not noticeably disintegrate. Dimers of NSs (circled in red color) were clearly
recognized in the sonicated structure. Interestingly, the SEM image in Figure 5.2.13 shows
different modes of the self-assembly of NS dimers, varying from their linear assembly to
staggered and side-by-side assembly.
Figure 5.2.13. Dark-field mode SEM image of self-assembled chains of single-patch gold
nanospheres following their 45 min sonication.Gold NSs were capped with PS-50K and
incubated in the DMF-water mixture at Cw=4 vol% at 40 ºC for 15 days. Dimers of NPs are
highlighted with red circles. Scale bar is 100 nm.
103
103
5.2.3.1 Self-assembly of patchy gold NSs in the presence of excess polymer.
The behavior of patchy NPs as "colloidal surfactants" was explored by examining their
self-assembly at the interface between two immiscible liquids (Figure 5.2.14a). 250 μL of the
mixed solution of gold NSs and non-thiolated (free) polystyrene with a molecular weight 50,000
g/mol in DMF was sonicated for 5 s. The concentration of the NSs and polystyrene were 0.3 nM
and 0.25 mg mL-1, respectively. Then, 250 μL of a DMF/water mixture at Cw=8 vol. % was
added dropwise to the NS-PS solution in DMF under gentle swirling of the vial, to reach Cw= 4
vol. %. The vial was sealed and maintained in a water bath at 40 °C for 24 h and the resulting NS
assemblies were subsequently imaged via TEM.
The amphiphilic nature of patchy NSs rendered them the ability to assemble at the
interface between immiscible liquids, thus reducing the surface energy of the system and
behaving as colloidal surfactants. Following the addition of water to the mixture of PS-capped
gold NSs and non-thiolated free PS molecules in DMF, the reduction in solvent quality led to the
formation of patchy NSs and PS-rich droplets. The NSs self-assembled on the droplet surface,
with a PS patch immersed into the droplet.
104
104
Figure 5.2.14. Self-assembly of patchy NSs in the presence of excess polymer. (a) Self-
assembly of gold NSs tethered with PS-50K on the surface of droplets enriched with free non-
thiolated PS (Mn=50,000 g/mol), after addition of water to Cw=4 vol. % and 1.25 nM solution of
polystyrene in DMF to the solution of the NSs. Scale bar is 250 nm. (b) Self-assembly of patchy
PS-50K-capped gold NSs in the DMF-water mixture at Cw=4 vol. % in the presence of 0.625 nM
of non-thiolated PS, following 5 min sonication of the solution. Scale bar is 40 nm.
A considerably higher energy of attachment of patchy NSs to liquid-liquid interface16, in
comparison with conventional Pickering emulsions, is expected to provide enhanced stabilization
properties of emulsions. Sonication of patchy NSs and non-thiolated PS in the DMF-water
solution led to the formation of elongated PS species decorated with patchy NSs (Figure
5.2.14b). These species were obtained after adding 500 μL of pre-formed single-patch 20 nm-
diameter gold NSs functionalized with PS-50K (prepared as described in Chapter 4) to a vial
containing 0.03 mg of polystyrene and sonicating the mixture 5 min. Following sonication, the
colloidal solution was immediately dropcast onto a TEM grid.
105
105
5.2.4 Self-assembly of surface-patterned nanocubes
Exposure of patchy gold NCs overnight to the DMF-water mixture at Cw =4 vol.% at 40
ºC resulted in their self-assembly. Figure 5.2.15 a-c shows that surface-patterned NCs formed
bonds via polymer patches formed at the NC edges (ribs), thus leading to the formation of open
structures. In contrast, the NCs uniformly coated with a highly dense polymer layer (non-patchy
NCs) upon their exposure to the DMF-water mixture self-assembled in short chains, with a face-
to-face bonding of the NCs to maximize the polymer contact and screen a greater polymer
surface area from the poor solvent, in agreement with our earlier work2. Figure 5.2.15 d-e shows
assemblies of silver NCs uniformly coated thiol-terminated polystyrene with a molecular weight
5,000 g/mol. We note that the formation of linear chains, as opposed to three-dimensional
clusters, was favored due to the strong contribution of electrostatic repulsion between the NCs at
Cw=20 vol. %1. As expected, in a good solvent, non-patchy NCs uniformly coated with PS-50K
formed close-packed structures with face-to-face NC contacts (Figure 5.2.15f), due to the
capillary forces acting between the NCs upon drying.
106
106
Figure 5.2.15. Self-assembly of surface-patterned and non-patterned nanocubes. In a-b and
C gold NCs were capped with PS-50K and incubated for 12 h at 40 ºC in the DMF/water
solution at Cw=4 vol.%. The ligand exchange procedure was carried out at PS-50K
concentration of 0.005 (a, b) and 0.5 mg/mL (c) in THF. Images d-e show face-to-face
assembled chain structures of silver NCs capped with thiol-terminated polystyrene with a
molecular weight of 5,000 g/mol after 2 h incubation in the DMF/water solution at Cw=20 vol %
at room temperature. In (e) gold NCs uniformly capped with PS-50K and incubated in THF for
12 h at room temperature formed close-packed structures with face-to-face NC contacts. Scale
bars are 100 nm.
A new binding modality was also observed for patchy nanocubes undergoing self-
assembly in an open, checkerboard structure, due to the binding of NC ribs in a poor solvent
(Figure 5.2.16), in marked difference with the face-to-face assembly of the nanocubes uniformly
coated with PS ligands. For patchy nanocubes, the face-to-face and the checkerboard assembly
via the formation of four bonds between the ribs would result in a similar reduction in the surface
107
107
free energy of the system, while for non-patchy nanocubes, the formation of close-packed
structures would be favored, due to the maximum screening of unfavorable polymer interactions
with a poor solvent2.
Figure 5.2.16. Self-assembly of patchy NCs functionalized with PS-50K in an open
checkerboard structure. Scale bar is 100 nm.
5.3 Conclusions
The nanopatterning approach using pinned micelle formation was validated for
nanoparticles with complex curvature profiles including spherocylindrical NRs, NRs with a
dumbbell shape, and NCs and triangular prisms. Pinned micelle formation was favoured on
regions of high curvature such as vertices, edges, and spherocylindrical tips in agreement with
theoretical predictions. In addition, pinned micelle formation was demonstrated on PS-capped
NCs in other solvents such as THF-water mixtures, demonstrating the flexibility in solvent
choice. Different end-thiolated polymers capping spherical gold NSs were also employed to
demonstrate the generality of the surface segregation approach, as it may be extended to many
polymers when solvent and polymer grafting density parameters are judiciously chosen. To this
end, gold NSs functionalized with (poly(4-vinyl pyridine) or (poly(N-vinyl carbazole)) displayed
108
108
qualitatively similar results to NSs functionalized with PS when subjected to poor solvency
conditions for the polymer ligands. Lastly, the self-assembly of patchy NSs and NCs indicate
that assembly modalities accessible to patchy NPs offers a promising route to stabilizing the
interfaces of immiscible liquids and non-equilibrium structures as well as forming new complex
structures such as open checkerboards. Further exploration is required to understand what
governs patch formation on complex nano-substrates as well as to control the patchiness which is
underway.
5.4 References
1. Choueiri, R., Klinkova, A., Thérien-Aubin, H., Rubinstein, M. & Kumacheva, E. Structural
transitions in nanoparticle assemblies governed by competing nanoscale forces. J. Am. Chem.
Soc. 135, 10262 (2013). DOI: 10.1021/ja404341r
2. Klinkova, A. et al. Structural and optical properties of self-assembled chains of plasmonic
nanocubes. Nano Lett. 14, 6314–6321 (2014). DOI: 10.1021/nl502746h
109
Chapter 6 Conclusions and Outlook
6.1 Summary and Conclusions
The work presented in this thesis spans the fields of nanoparticle synthesis, surface
functionalization, and self-assembly, polymer physical chemistry and colloidal chemistry. The
findings demonstrate the interplay of all these disciplines in working towards functional
materials.
In Chapter 3, the self-assembly of nanospheres isotropically functionalized with polymer
ligands was explored. It was shown that when accounting for interparticle forces at the
nanoscale, assembly of the same building blocks yields different self-assembled structures. In
particular, by tuning the molecular weight of the polymeric ligands or by varying the polarity of
the solvent the self-assembled structures transitioned from a regime where electrostatic repulsion
dominated and yielded one-dimensional chain-like structures to a regime where hydrophobic
interactions dominated generating globular structures. The experimental results were validated
using theoretical predictions using calculations of the overall free energy of the self-assembled
structures in different solvents. It was confirmed that polar solutions favour chain-like
assemblies, while less polar media or addition of salt favour the formation of globular
assemblies. We demonstrated the reversibility of the self-assembled structures and found that
disassembly required energy input to break polymer-polymer physical bonds. Lastly, we
characterized the terminal globular size and showed hierarchical structures made from
assemblies of globules into globule networks opening the door for future research in hierarchical
assemblies and potentially metamaterials using analogous approaches.
110
110
In Chapter 4, thermodynamically driven segregation of polymeric brushes on the surface
of gold nanospheres into pinned micelles was studied in poor solvents. When the footprint area
of a pinned micelle was comparable with the nanosphere area, nanoparticles with a controllable
number of polymer patches were formed. For the same polymer grafting density, this effect was
favored for smaller nanospheres, that is, nanoparticles with a larger curvature. The generation of
single and multi-patch nanospheres was explored and statistical analysis of the generated
structures revealed close agreement with the theoretical model. Photocrosslinking of the surface
polymer patches was achieved, which allowed us both to confirm the formation of patches in
solution and optionally, to permanently preserve the patchy structure. Tomography was
performed on the structures to gain an understanding of the three-dimensional distribution of
polymer on the surface of the nanospheres. The results of this study resulted in a new
methodology for generating patchy nanocolloids. It also furthered understanding of the
behaviour of surface-capping polymers in different solvent conditions.
In Chapter 5, to show the versatility of this approach, surface segregation of polymer
ligands in pinned micelles was explored on nanoparticles with different shapes and
compositions, as well as for nanoparticles coated with different polymer ligands in response to
different external stimuli changing the solvent quality. Polymer segregation on gold nanorods
with spherocylindrical and nanodumbbell shapes, as well as nanocube ribs confirmed the
theoretically predicted preference for polymer segregation on regions of high curvature. Self-
assembly of patchy nanospheres and nanocubes revealed new self-assembly modes; inaccessible
to nanocolloids uniformly coated with polymer ligands. In particular, patchy nanospheres were
shown to behave as colloidal surfactants, decorating the surface of polymer droplets in solution,
while patchy nanocubes assembled in an open checkerboard structure. The findings in this
111
111
chapter demonstrate the versatility of the nanopatterning approach and provide a framework for
further adaptation of the methodology to new polymer-nanoparticle systems.
In summary, we have demonstrated the versatility of polymer-functionalized
nanoparticles both in their isotropically functionalized and patchy states as building blocks for
self-assembly and potential templates for nanopatterning. In studying the behaviour of polymer
ligands on nanocolloids we touch on fundamental themes relevant to many fields and
applications, e.g. surface coatings, anti-fouling and in vivo applications and have opened the door
for further studies on the morphology of tethered polymers on rough substrates. For these
reasons, our aim in exploring the parameter spaces of polymer ligand properties and nanoparticle
properties was to develop an understanding of the role of material properties on the resultant
structures and to open the door for rational design of these systems geared to specific
applications.
6.2 Outlook
The work undertaken in this thesis has explored new territory in the self-assembly and
surface functionalization of colloidal nanoparticles. Each chapter opens up new potential projects
and avenues of research.
In Chapter 3, the self-assembly of isotropically-functionalized nanoparticles into different
structures based on the interplay of nanoscale forces opens up potential research directions in the
fine-tuning of interactions between nanocolloids, in order to make switchable structures
including vesicular, chiral, and hierarchical assemblies, thus yielding multi-functional systems
for drug delivery and sensing. Due to the facile and adaptable surface functionalization protocol,
many functional polymers may be employed including biocompatible, anti-fouling, conductive,
112
112
and amphiphilic polymers allowing for the integration of these nanocolloids in different
environments and applications.
In Chapter 4, the surface segregation of polymer ligands lends itself as a template for the
nanopatterning of the surface of nanocolloids. For instance, patterned heterostructures may be
formed by reduction of metals in the polymer-deprived surface regions, yielding new building
blocks with complex morphologies and properties. In addition, different species may be
selectively sequestered in polymer patches and subsequently, released upon reconfiguration of
polymer ligands. The patchy particles may then be used as probes, drug delivery agents or
potentially as nano-reactors potentially utilizing the properties of the nanoparticle core (plasmon-
enhanced catalysis or sensing) to aid in chemical transformations. Fundamentally, the study of
polymer ligand morphology is important in determining the fate of nanoparticles in vivo and an
understanding of the behaviour of polymer ligands on rough surfaces or biologically relevant
polymer ligands on nanoparticle surfaces is another potential avenue of exploration.
In Chapter 5, the adaptation of the polymer segregation approach on nanoparticles with
different shapes lends itself to further possibilities in hetero-nanostructure synthesis. In
particular, the complex polymer segregation behaviour on spherocylindrical nanorods potentially
points to continuous ribbons of segregated polymer possibly yielding chiral nanostructures.
Work is currently underway to confirm this hypothesis. The polymer segregation on other
nanoparticles, such as nanocubes allows for the exploitation of shape-specific plasmonic
properties on reactive chemical species or targeted analytes sequestered in polymer patches and
may allow for studies of plasmon enhancement originating specifically from nanocube tips. The
self-assembly of patchy nanocolloids in e.g., dimers and trimers may also lead to new projects of
interest, including the fine-tuning of interparticle plasmonic hot-spot geometry via patchy
113
113
nanoparticle assembly. On the other hand, assembly of patchy nanoparticles at the interface of
two immiscible liquids also requires further exploration, especially, with respect to stabilizing
kinetically trapped structures.
In summary, more work is required to fully understand how polymer-functionalized
nanoparticles may be used to generate dynamic, hierarchical, and complex self-assembled
structures using functional polymer ligands. In addition, more work is required to understand and
characterize polymer segregation behaviour on nanoparticles with different shapes, as well as to
adapt these patchy nanocolloids to fabricate new complex heterostructures. Lastly, the
application of these patchy nanocolloids as probes or mini-reactors may be a fruitful avenue of
further research.
114
114
Appendix 1: Derivation of calculation of nanoscale forces involved in self-assembly of PS-coated gold NPs
Determination of the interfacial tension between PS-DMF/water and DMF/water phases
Direct measurement of the interfacial tension, PS/L, between the PS-DMF/water and
DMF/water phases (denoted as “PS” and “L”, respectively) was not possible, due a close-to-
glassy state of the collapsed PS molecules in the DMF/water mixture. Instead, the value of PS/L
was calculated from the individual values of surface tension of the PS-phase, PS, and the L-
phase, L, by using the extended Fowkes equation1,2 as
γPS/L = γPS + γL -2 (γPSd γL
d )1/2- 2 (γPSp γL
p)1/2 (A1),
where the superscripts d and p refer to the dispersive and polar components of the surface
tension.
To determine the dispersive component of γL, the sessile drop method was used to
measure the contact angle of the liquid on a solid substrate, which only had dispersive
interactions with the liquid. The contact angle, , was then related to the dispersive component
of γL by using the Owens-Wendt equation as3
(A2)
where the dS is the dispersive component of the surface tension of the substrate used. In our
work, we used poly(ethylene terephthalate) (PTFE) as the solid substrate with Sd
=19.0 mN/m
(the polar component of the surface tension for PTFE was zero).4,5 The value of γLp was then
115
115
found as γL-γLd. The measured contact angles, the values of surface tension and the values for
the dispersive component of the surface tension for the DMF/water solutions at Cw of 5 and 15
vol. % (the L-phase) are summarized in Table A1.
The surface tension of the PS-DMF/water mixture, PS, was calculated to be 38.4 and 40.8
mN/m for Cw of 5 and 15 vol. %, respectively,6,7 with polar components of 10.2 and 12.7 mN/m
and dispersive components of 28.2 and 28.1 mN/m. The values of the interfacial tension, PS/L,
calculated using (
A1 are given in Table A1.
Table A1. Surface properties of the “PS” and “L” phases
Cw
(vol. %)
L
(mN/m)
(o)
L d
(mN/m)
L p
(mN/m)
PS
(mN/m)
PS d
(mN/m)
PS p
(mN/m)
PS/L
(mN/m)
5 37.1 87.8 20.6 16.5 38.4 28.2 10.2 1.4
15 46.3 97.2 22.8 23.6 40.8 28.1 12.7 1.9
Calculations of the change in energy following NP assembly
Figure A1 shows schematically fragments of self-assembled nanostructures for three adjacent
NPs. Based on analysis of TEM images, for the NPs functionalized with PS-50K, the radius of
the gold core, R, was 11.5 nm; the total radius, Rt, of the NP with a gold core and the PS “shell”
was 15.5 nm; and the center-to-center distance, D, between two adjacent NPs was 28.0 and 29.5
nm for Cw of 5 and 15 vol. %, respectively (corresponding to the thickness, d, of the PS layer
confined NP surfaces of 5 and 6.5 nm, respectively). The angle between the lines connecting
116
116
the center of the second NP with the centers of the first and the third NPs varied from 180o
(chain configuration, Figure A1a) to 60o (globular configuration, Figure A1c). The distance, L,
between the centers of the first and the third NPs changed from 4R+2d to D=2R+d for the chain
and globular configurations, respectively.
Figure A1. Schematics of NP assembly and definition of variables describing NP
configuration. (a-c) Assembly of three NPs in the chain configuration (=180o) (a), intermediate
configuration (b) and globular configuration (=60 o) (c). R is the radius of the metal gold cores
and Rt the radius of the particle with the PS shell.
When two NPs associated, the fragments of their surface that were originally exposed to
the solvent formed contact (shown as a biconvex lens in Figure A2), thereby screening a part of
PS shell from unfavorable interactions with a poor solvent and reducing the total surface energy
of the system.
R
Rt DD
D
L
θ
D
D
L=D
(a) (b) (c)
θ
d
117
117
Figure A2. Schematic of the formation of the interface between two associating NPs.
Overlap and adhesion of the PS shells lead to the reduction of the area of NP surface exposed to
the poor solvent.
The surface area of the ensemble of two NPs exposed to poor solvent decreased as shown
in Figure A2 by
A 4 Rt L
2
Rt (A3),
where L is given by
L 2D2 1 cos (A4)
and the notations are explained in Figure A1.
118
118
The change in attractive surface tension energy was calculated for two NPs coated with
PS-50K in the DMF/water mixture at Cw of 5 and 15 vol. %, when they made a contact, with a
reduction of the surface area A.
Est= PS/L A (A5)The values of interfacial tension PS/L were 1.36 10-2 J/m2 and 1.94 10-2 J/m2 for
the DMF/water compositions with Cw of 5 and 15 vol. %, respectively (Table A1). At large
separations (or > 64 o), the NPs did not make contact and the surface area did not change
(A=0), yielding Est=0. Red lines in Figure A3 show that for Cw=5 vol %, the reduction in Est
upon contact of NPs 1 and 3 was significantly stronger than for Cw=15 vol. %.
Figure A3. Variation in surface tension energy and electrostatic energy between two
associating NPs as a function of angle . (a,b) Variation in surface tension energy (red lines)
and electrostatic energy (black lines) between two associating NPs, when angle (as in Figure
A1) changes from 60 to 300o in the DMF/water solution at Cw = 5 vol. % (a) and Cw = 15 vol. %
(b).
119
119
The change in electrostatic energy, Eel, was calculated using the approximation
developed for charged spherical NPs functionalized with polymer brushes8 as a logarithmic
function of the surface-to-surface interparticle distance d=L-2R with respect to this distance 2D-
2R, in the chain configuration (=180o). By integrating the electrostatic force between particles,
Rdl
FB
2kT
2
(Equation 28 in Reference 9, the case of large interparticle separation, since L >
Rt) over distance d from L-2R to 2D-2R, the following expression was obtained:
RL
RDR
lE
2
22ln
kT
2 B
2
el
(A6),
where k is Boltzmann’s constant, T is the temperature, and lB is the Bjerrum length. For
DMF/water solutions at Cw of 5 and 15 vol. %, we calculated lB to be 1.4 and 1.14 nm,
respectively (see Chapter 3). Equation A5 uses the notations introduced in Figure A1, where R is
the radius of the gold core of the NP, D is the center-to-center distance between the two
neighboring PS-coated NPs and L is the center-to-center distance between the 1-st and the 3-rd
PS-coated NPs (equal to 2D and D for of 180 and 60o, respectively). The variation in
electrostatic energy between the 1-st and the 3-rd NPs was calculated for the range 60 300o
and plotted in Figure A3 for Cw of 5 and 15 vol. %.
The variation of the total energy of the system was deduced as
ΔEt = ΔEel + ΔEst (A7)
and calculated separately for two cases as
Et
4PSL Rt L
2
Rt
2
2
kT
lB
R ln2D 2R
L 2R
if L 2Rt
2
2
kT
lB
R ln2D 2R
L 2R
if L 2Rt
(A8)
120
120
The summary of the calculations of changes in repulsive electrostatic energy and attractive
surface tension energy, as well as the change in total energy of the system, is given in Table A2
for two solvent compositions and two different angles, corresponding to the globular and chain
NP configurations.
Table A2. Summary of the changes in electrostatic energy, surface tension energy and the
total energy for the system containing three NPs in =60o configuration.
Cw = 5 vol. % Cw = 15 vol. %
∆Est (kT) -96 -69
∆Eel (kT) 76 85
∆Et (kT) -20 16
Figure A4 shows the variation in the total energy of the system, Et, with angle
(defined in Figure A1) for two solvent mixtures. For CW=5 vol.%, the deepest minimum in Et
corresponded to =60o (globular configuration of NPs), while for CW=15 vol.%, the lowest
minimum in Et occurred at =180o (chain configuration of NPs ). Both predictions were in
agreement with experimental results. Furthermore, in NP chains the fraction of 180 o angles
(binned as 180+/-60 o) was 80%.
121
121
Figure A4. Variation in the total energy of interactions between two PS-coated gold NPs,
plotted as a function of angle (as described in Figure A1 for CW = 5 vol. % (a) and CW = 15
vol.% (b).
122
122
References
1. (a) Fowkes, F. M. Determination of interfacial tensions, contact angles, dispersion forces in
surfaces by assuming additivity of intermolecular interactions in surfaces. J. Phys. Chem. 66,
382–382 (1962). DOI: 10.1021/j100808a524 (b) Li, I. T. S. & Walker, G. C. Interfacial Free
Energy Governs Single Polystyrene Chain Collapse in Water and Aqueous Solutions. J. Am.
Chem. Soc. 132, 6530–6540 (2010). DOI: 10.1021/ja101155h
2. Israelachvili, J.N. Intermolecular and Surface Forces. Acad. Press: San Diego, CA, 2011.
3. Owens, D. K. & Wendt, R. C. Estimation of the surface free energy of polymers. J. Appl.
Polym. Sci. 13, 1741–1747 (1969). DOI: 10.1002/app.1969.070130815
4.Wu, S. Polymer Interface and Adhesion. Marcel Dekker: New York, 1982.
5. Żenkiewicz, M. Methods for the calculation of surface free energy of solids. J. AMME. 24,
137–145 (2007).
6. Ober, R., Paz, L., Taupin, C., Pincus, P. & Boileau, S. Study of the surface tension of polymer
solutions: theory and experiments. Good solvent conditions. Macromolecules 16, 50–55 (1983).
DOI: 10.1021/ma00235a010
7. Calculated for a mixture containing PS (36 vol. %) and DMF/water solution (64 vol. %).
8. Zhulina, E. B., Boulakh, A. B. & Borisov, O. V. Repulsive Forces between Spherical
Polyelectrolyte Brushes in Salt-Free Solution. Zeitschrift für Phys. Chemie 226, 625–643 (2012).
DOI: 10.1524/zpch.2012.0279
9. Figuerola, A. et al. End-to-End Assembly of Shape-Controlled Nanocrystals via a
Nanowelding Approach Mediated by Gold Domains. Adv. Mater. 21, 550–554 (2009). DOI:
10.1002/adma.200801928
123
Appendix 2: Theoretical derivation of patchy nanoparticle formation
Theoretical analysis of the state of the polymer layer on the nanosphere surface in a poor solvent
Consider a nanosphere NS with a diameter D that is capped by flexible polymer molecules
end-tethered to its surface with a grafting density . Each polymer molecule is composed of N>>1
Kuhn segments with a Kuhn length b on the order of the monomer size. The poor quality of the
solvent for the polymer ligands is quantified by the dimensionless excluded volume parameter =
(T - )/T, where T and are the absolute temperature of the experiment and the theta-temperature,
respectively and 0 < < 1. An alternative characterization of this poor solvent is given by the size
≈ b-1 of thermal blobs that attract each other with thermal energy kT, where k is the Boltzmann
constant1. The number of these thermal blobs per molecule is N2 > 1. An isolated polymer
molecule in a poor solvent forms a spherical globule with a radius R0 ≈ b(N/)1/3. The surface
tension at the polymer-solvent interface of this globule is 2 ≈ kT2/b2. We assume that the surface
tension 1 at the NS-solvent interface is equal to the surface tension 3 at the NS-polymer interface,
giving the contact angle 90o for the polymer globule on the NS surface, independent of the NS
diameter. Polymer volume fraction inside the globule is ϕ ≈ < 1.
Diagram of the states of polymer molecules end-grafted to the nanosphere surface in a poor solvent
Figure A5 shows a theoretical diagram of the states of the polymer end-grafted to the
surface of a NS in a poor solvent. Different states of this polymer are described below as different
regimes. Experimentally relevant part of the diagram (outlined by a rectangular purple dashed line)
is described briefly in the present section and discussed in detail in the following section.
124
124
Figure A5. Theoretical diagram of states of the NS functionalized with end-tethered
polymer.The diagram is plotted in double logarithmic coordinates. Parameters used: N=70, τ =
0.35, b=1.8 nm, contact angle 90o. Experimentally relevant part of this diagram is outlined with
the dashed rectangle. The cartoons illustrate different states of the polymer on the surface of NSs.
Regime Is. The diagonal line with a slope -1/2, described by the equation D2 = 1,
gives the condition of grafting single polymer molecule per NS. The regime Is below this line
corresponds to the low grafting density with on average of less than one polymer molecule per
NS.
Regime Im. Above the red section of the diagonal line in the regime Im, the NSs are capped
with more than one chain. At a low grafting density ( <b-2N-4/3 -2/3), several single-chain polymer
globules (patches) do not coalesce, since the stretching energy penalty kTτ/(bσ1/2) is higher than
the gain in surface energy kTτ4/3N2/3. As a result, grafted polymers form on the NS surface
individual globules (patches). The number of globules/NS is D2. The section of red line with a
slope -1/2 between the regimes Im and Is corresponds to the boundary between NSs with a different
average number of single-chain patches.
3 2 1 00
0.5
1
1.5
22
0.46
1.5
1.08
0.92
0.46
2
2
1.85
0.93
1.39
03 3 2.16 1.38 0.91 3 0 0 0 0.91
V
IIIa
VI
Ia
IIIII
Is
N1/2
IV
N1/2 2N4/32/3
Im
100
N 2
N
1Na
no
sp
he
red
iam
ete
r, D
/b
Grafting density, b2
101102103100
102
VII
125
125
Regime Ia. When the NS is smaller than the thermal blob (D < ), it interacts with segments
of polymer molecule with size up to D. These segments are too small to “feel” the poor quality of
the solvent. This small NS is fully engulfed with the polymer globule. The boundary between the
regimes Is and Ia corresponds to D ≈ b/.
Regime II. Groups of P neighbouring tethered molecules segregate into pinned micelles
with a core comprising the majority of chain monomers and the stretched micellar legs attaching
the core to the NS surface. The average number of pinned micelles (patches) per NS, D2/P,
decreases with decreasing NS diameter D and approaches unity through a cascade of transitions
between NS with a different average number of micelles (thick red line between regimes II and
VII).
Regime III. At a higher grafting density >/(b2N1/2), the pinned micelles merge into a
laterally uniform layer with polymer concentration . In this regime, the distance between the
grafting points 1/2
is larger than the thermal blob size ξ ≈ b/. The thickness of the uniform
polymer layer H ≈ b3
N/is smaller than the diameter D of large NS.
Regime IIIa. Below the dashed line D = b
3
N/ with a slope 1, the uniform polymer layer
is thicker than the NS diameter. The stretching of the tethered molecules becomes non-uniform
and decreases with the distance r from the NS center2. The local chain stretching near the NS
surface disappears at the boundary D = /(b) between the regimes IIIa and VII. At this boundary,
the penalty to form a leg of a pinned micelle (a string of D/b thermal blobs) becomes comparable
to the gain in energy due to the formation of the interface between micelle core and NS surface.
126
126
Regime IV. At the grafting density of chains > (τ/b)2, the distance between the grafting
points -1/2 becomes smaller than thermal blob size ξ ≈ b/ and the grafted polymer chains repel
each other via three-body interactions. The planar-like -brush3 has the polymer layer thickness
H ≈ bN(b2)1/2, which is smaller than the NS diameter D.
Regime V. The NS diameter D is smaller than the polymer layer thickness, D < H, below
the boundary D = bN(b2)1/2 between the regimes IV and V. The planar layer transforms into a
spherical star-like -layer with the thickness H ≈ bD1/2(b2)1/4N1/2 and tension decreasing with the
distance from the NS center2. The outer tension blob becomes equal to the thermal blob at the
boundary between the regimes V and VI for D = N-1/22.
Regime VI. The polymer layer on smaller NS with D< N-1/22 consists of the collapsed
outer sublayer with the thickness H ≈ b[D2(b2)N/]1/3 and the inner -sublayer with the thickness
r ≈ b-1/2-1D-1.20 The -sublayer disappears when its thickness becomes comparable to the size
of the thermal blob r ≈ b-1, which corresponds to the boundary D ≈ -1/2 between regimes VI and
Ia.
Regime VII. The NS carries a single pinned micelle composed of D2
molecules for D
< /(b) (below regime IIIa) and for D > -1/2 (above regime Is).
Theoretical analysis of experimentally relevant polymer states on the nanosphere surface
Figure A6A represents a fragment of the theoretical diagram highlighted with a dashed
rectangle in Figure A5, and includes Regimes II, III (including IIIa), and VII. At temperature T,
the polymer separates from the solvent into a relatively dense phase with the polymer volume
127
127
fraction ϕ ≈ τ, where τ is the dimensionless excluded volume parameter of the polymer1. The
diagram plotted in the σ-D coordinates contains two regions, namely, a pinned micelle regime II
(patchy NSs) and a smooth shell regime III (core-shell NSs), which are separated from each other
with a blue boundary line. The cartoons in Figure A6A illustrate NSs with a different number of
surface patches.
Figure A6. Theoretical and experimental diagrams of states of polymer-grafted
nanospheres. (A) A theoretical diagram of states of polymers grafted with density σ to the
surface of a NS with diameter D. The upper right part of the diagram (regime III for high σ and
large D) corresponds to core-shell NSs. The rest of the diagram (regimes II and VII) represents
pinned micelles with the number n of micelles per NS increasing from the bottom right to the top
left of the diagram. The dark-blue line (eq. S11) shows the boundary between the NSs with a
smooth polymer layer (regime III) and patchy NSs (regimes II and VII). Red lines (eq. S9)
indicate cascade of transitions between the NSs with a different number n of pinned micelles.
The cartoons illustrate NSs with a different number of surface patches. (B) Experimental
10
0.001 0.010 0.100
D(n
m)
(chains/nm2)
20
40
80
100 Core-shell NSs
Patchy NSs
B
1 103
0.01 0.1 1
10
100100
10
x
z1 x3 3 ( )
z1 x2 2 ( )
z1 x1 1 ( )
80
60
40
20
10
10
10
10
10
10
10
10
0.41 10
3
y1 x( ) x3 x2 x1 0.001 0.001 0.001 0.001 0.4 0.002 0.004 0.006 0.008 0.02 0.04
(chains/nm2)
D(n
m)
40
20
Patchy NSs
Core-shell NSs
n=1
n=2
n=4A 100
80
60
100.010 0.100 0.001
n=3
10
0.001 0.010 0.100
D(n
m)
(chains/nm2)
20
40
80
100 Core-shell NSs
Patchy NSs
B
1 103
0.01 0.1 1
10
100100
10
x
z1 x3 3 ( )
z1 x2 2 ( )
z1 x1 1 ( )
80
60
40
20
10
10
10
10
10
10
10
10
0.41 10
3
y1 x( ) x3 x2 x1 0.001 0.001 0.001 0.001 0.4 0.002 0.004 0.006 0.008 0.02 0.04
(chains/nm2)
D(n
m)
40
20
Patchy NSs
Core-shell NSs
n=1
n=2
n=4A 100
80
60
100.010 0.100 0.001
n=3
10
0.001 0.010 0.100
D(n
m)
(chains/nm2)
20
40
80
100 Core-shell NSs
Patchy NSs
B
1 103
0.01 0.1 1
10
100100
10
x
z1 x3 3 ( )
z1 x2 2 ( )
z1 x1 1 ( )
80
60
40
20
10
10
10
10
10
10
10
10
0.41 10
3
y1 x( ) x3 x2 x1 0.001 0.001 0.001 0.001 0.4 0.002 0.004 0.006 0.008 0.02 0.04
(chains/nm2)D
(nm
)
40
20
Patchy NSs
Core-shell NSs
n=1
n=2
n=4A 100
80
60
100.010 0.100 0.001
n=3
A
II
III
VII
128
128
diagram of states of gold NSs tethered with PS-50K, plotted in the D- coordinates. The dark-
blue boundary line outlines conditions, at which core-shell NSs () and NSs with 5 (), 4 (),
3 (), 2 () and 1 ( ) patches are formed. Insets show a core-shell NS (=0.03 chains/nm2)
and a patchy NS (=0.002 chains/nm2).
Right region of the state diagram (Regime III). At high grafting densities σ (to the right of
the boundary line), the grafted polymer chains are stretched to a height exceeding the unperturbed
size of the polymer molecule, R=bN1/2, where N is the number of Kuhn segments of size b per
polymer chain1. Deviations of the shape of this layer from a smooth spherical shell are unfavorable,
because they increase the area of the polymer-solvent interface in a poor solvent and thus the
interfacial energy of the system. Such deviations also increase the polymer stretching free energy4.
As a result, the dense polymer phase forms a uniformly thick shell around the NS.
Transition between the right and left parts of the diagram. Next, we consider the effect of
NS dimensions on the transition between the smooth layer and the pinned micelle regimes. For
large NSs (D >> R), the transition occurs at polymer grafting density
σ ≈ τ/(bR) (A9),
at which the layer thickness is comparable to an unperturbed polymer size R. For large NSs, at a
lower σ, the polymer molecules in the layer are no longer extended. To reduce the interfacial
energy, the condensed polymer phase undergoes dewetting, that is, the segregation of the smooth
layer into small droplets (corresponding to the cores of pinned micelles). This effect results in the
exposure to the solvent of the NS surface with a lower NS-solvent interfacial tension γ1 than the
tension1 γ2 ≈ kTτ2/b2 of the polymer-solvent interface. The dewetting is constrained, because the
129
129
polymer molecules (micellar “legs”) are strongly surface-tethered and have to stretch to reach
micellar cores.
Each pinned micelle is composed of P chains, with the micellar core comprising the
major fraction of each polymer molecule, and P legs. The aggregation number P of the micelles
is determined by the balance of the surface free energy per polymer chain ~ γ2Rc 2/P of the
micellar core of radius Rc and the free energy of stretched legs. The average length of a leg is
given by the radius A1/2≈(P/σ)1/2 of a footprint area A of the micelle. The stretching energy of the
leg can be estimated as kT(P/σ)1/2τ/b, that is, the ratio of the leg length and a characteristic
tension length scale b/τ, dependent on the quality of the solvent. Since the micellar core has the
same polymer volume fraction ϕ ≈ τ as the dense polymer phase, its radius Rc is related to the
aggregation number P as Rc ≈ b(NP/τ)1/3. The balance of the interfacial free energy per polymer
chain in the core, γ2𝑅c2/P (≈ kTτ2(NP/τ)2/3)/P) and the free energy of leg stretching, kT(P/σ)1/2(τ/b),
determines the micellar aggregation number of large NSs in a quasi-planar regime5.
P ≈ N4/5(σb2)3/5τ2/5 (A10)
The incline of the blue boundary for the NSs with large but finite diameters D > R is governed
by the curvature-induced increase of the area of polymer-solvent interface per chain with a
decreasing NS diameter at σ = constant, that is, from 1/σ per chain to (1+2H/D)/σ per chain, where
H is the thickness of the polymer layer. The corresponding increase of the surface free energy per
chain 2γ2H/(Dσ) destabilizes the uniformly thick polymer shell. At a particular grafting density σ
and decreasing NS diameter D, this effect is stronger than the increase of free energy of micellar
legs, due to their extra-stretching proportional to 1/D2. Therefore, a uniformly thick polymer layer
130
130
on smaller NSs is destabilized at larger values of σ, thus resulting in the incline of the blue
boundary in Figure A6A for D > R toward larger grafting density
𝜎
𝑏(
1
𝑅+
2
𝐷) (A11),
where the sum in the parenthesis is the sum of the reciprocal chain size 1/R and the reciprocal NS
radius 2/D.
Left region of the state diagram (Regimes II and VII). At a lower grafting density σ (to the
left of the blue boundary line), the NSs are covered by pinned micelles. The number of pinned
micelles
𝑛 ≈𝐷2𝜎
𝑃≈ (
𝐷
𝑏)
2
(𝜎𝑏2
𝜏𝑁2)
2 5⁄
(A12)
and the micellar size (footprint diameter) (P/σ)1/2 are not strongly affected by the NS curvature,
as long as D >> (P/σ)1/2. This condition is equivalent to n >> 1, that is, a large number of
micelles per NS for
D >> bN2/5 [τ/(σb2)]1/5 (A13)
A decrease in D leads to the decrease in the total number, n, of pinned micelles per NS
(Equation A12) without a noticeable change in their equilibrium aggregation number P, as long as
n >> 1. Transitions between the NSs with a different integer number n of pinned micelles are
illustrated by the series of red lines in the diagram in Figure A6A. Each of these lines between NS
states with n and n +1 micelles is described by the relation
𝐷 ≈ [(𝑛 +1
2)
𝑃
σ]
1 2⁄
(A14)
131
131
Using Equation A10 for equilibrium aggregation number, P, one obtains the expression for the
red transition lines in Figure A6A
𝑫 ≈ (𝒏 +𝟏
𝟐)
𝟏 𝟐⁄
𝒃𝑵𝟐 𝟓⁄ (𝝉
𝛔𝒃𝟐)𝟏 𝟓⁄
(A15)
The transition between a single-patch and two-patch NS (that is, a NS with one and two
pinned micelles, respectively) is illustrated in Figure A7. The transition occurs when the legs of a
single- patch NS are over-stretched, while the legs on a two-patch NS are not sufficiently stretched
to balance interfacial energy of micellar core.
Figure A7. Schematic of the transition between a single-patch and a two-patch
nanospheres. The NSs coexist at the transition line (Equation A15) with n=1. Polymer chains
are shown by dark green.
In Figure A6A, below the lowest red line (corresponding to a single pinned micelle per NS,
n = 1), the decrease in D leads to the reduction in the total number ~D2σ of tethered chains and the
corresponding micelle aggregation number P ≈ D2σ. The surface free energy gain due to partial
NS wetting by the micelle core (compared to the complete NS wetting with a laterally
homogeneous layer) decreases as γ2D2. This gain is balanced by the free energy loss of kTDτ/b per
stretched leg. For small NSs at D < R, the position of the blue line in Figure A6A is determined
by the balance of the interfacial surface energy ~ γ2D2 ≈ kTτ2D2/b2 and the free energy of D2σ
stretched legs losing ~ kTDτ/b per leg. This boundary between the laterally smooth layer (regime
III) and the single pinned micelle regime VII is given by
Increase D and/or increase σ
Increase D and/or increase σ
Decrease D and/ordecrease
Increase D and/orincrease
132
132
D ~ τ/(bσ) (A16)
with a slope of -1 on a double-logarithmic scale in the right lower part of the diagram in Figure
A6A, consistent with Equation A11 for R>>D. Therefore, the equation for the blue boundary in
Figure A6A given by Equation A11 can be rewritten as
𝐷 ≈2
𝑏𝜎
𝜏−
1
𝑅
(A17)
Although the scaling-type diagram of states in Figure A6A was constructed for a polymer-
NS contact angle of 90o, it is expected that the main features of the diagram would hold for
smaller contact angles reported in Section 2.2.4.
Comparison of the experimental and theoretical diagrams of NS states. The experimental
state diagram plotted in Figure A6B reproduced all essential features of the theoretical diagram
(Figure A6A), including the transition between the patchy NSs and core-shell NSs. Both diagrams
underlined that for small NSs (larger curvature) the transition occurred at higher grafting densities
than for large NSs. This trend is reflected by the negative slope of the blue boundary line at small
values of D and high values of . By inserting the experimental value of =0.35 for the DMF/water
mixture at CW=4 vol. %6, Kuhn length1 b=1.8 nm, chain size R=15 nm and without any adjustable
parameters in Equation A17, the shape and the position of the boundary line between the patchy
and smooth-shell NSs was reproduced (see blue lines in Figure A6 A and B).
For small and intermediate values of , the number of patches per NS, n, increased with
increasing NS dimensions. For a particular value of D, no obvious variation of n vs. was
observed, with an exception of 80 nm-diameter NSs for which n decreased with reducing (as
predicted by Equation A12). The transitions shown with red lines in Figure A6A were obtained by
inserting =0.35, b=1.8 nm, N=70 without using any adjustable parameters in Equation A15.
133
133
References
1. Rubinstein, M. & Colby, R. H. Polymer Physics. (Oxford University Press, 2003).
2. E. B. Zhulina, O.V. Borisov & T. M. Birshtein, Coil-globule transition in star-like
macromolecules. Polym. Sci. U.S.S.R. 30, 780–788 (1988).
3. E. B. Zhulina, O. V. Borisov, & V. A. Priamitsyn, Theory of steric stabilization of colloid
dispersions by grafted polymers. J.Colloid Interf. Sci. 137, 495–511 (1990).
4. F. J. Solis, G. T. Pickett, On the stability of the Alexander polymer brush. Macromolecules 28,
4307–4312 (1995).
5. E. B . Zhulina, T. M. Birshtein, V. A. Priamitsyn. & L. I. Klushin, Inhomogeneous structure of
collapsed polymer brushes under deformation. Macromolecules 28, 8612–8620 (1995).
6. Z. Nie, et al. Self-assembly of metal-polymer analogues of amphiphilic triblock copolymers.
Nat. Mater. 6, 609–614 (2007).
134
Copyright Acknowledgements